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STATE ESTIMATION: A DECISION THEORETIC APPROACH

By

YOAV N. LEVINBOOK

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007

@ 2007 Yoav N. Levinbook

To the memory of my father, Benjamin Levinbook

ACKNOWLEDGMENTS

I would like to thank Professor Tan F. Wong, my advisor and the chair of the supervisory

committee, for his guidance, useful advice, and, in particular, for the freedom and encouragement

he gave me for pursuing my own research interests. I feel that I significantly evolved as an

electrical engineer in the four years I worked closely with him. I have no doubt that without his

help I would not have completed this work.

I wish to take this opportunity to thank all the members of the supervisory committee for

their helpful comments and suggestions which have helped improve this work.

I would also like to thank Professor Paul Robinson and Professor Sergei Shabanov from the

Department of Mathematics, who were my instructors in several courses. The knowledge I have

gained from them proved to be very valuable for this work.

I would like to express my deepest gratitude to my beloved mother and late father, which

have always supported me and encouraged me. I hope I lived up to their expectations.

Finally, I am indebted to my dear wife, Eliane, for her support, encouragement, and

patience. Without her, I could not have confronted all the difficulties of the last four years.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ......... . . .. 4

LISTOFFIGURES ............. .............. 7

LIST OF ABBREVIATIONS ......... . .. .. 8

ABSTRACT.............. ......... ...... 9

CHAPTER

1 INTRODUCTION ......... ... .. 11

2 GENERAL NOTATION AND CONVENTIONS .... .... .. 17

3 DECISION THEORETIC FORMULATION ... .. . .. 19

4 GENERAL DECISION THEORETIC RESULTS ... .. .. 24

5 THE CASE THAT THE RISK IS SPECIFIED BY A LOSS FUNCTION .. .. .. 34

6 THE CASE OF CONVEX LOSS FUNCTION .... .... .. 40

7 FINDING A MINIMAX ESTIMATOR AND THE DUAL PROBLEM .. .. .. .. 49

8 APPROXIMATING A MINIMAX ESTIMATOR .... .... .. 52

9 THE RESTRICTED RISK BAYES PROBLEM AS A MINIMAX PROBLEM .. 61

10 ESTIMATION WITH A RESTRICTION ON THE OBSERVATIONS THAT CAN

BEUSED ............. ............... 66

11 THE STATE ESTIMATION PROBLEM . ... .. 73

12 AFFINE STATE ESTIMATION BASED ON QUADRATIC LOSS FUNCTIONS .. 77

12.1 Finding a Restricted Risk Bayes Solution ... .... .. 77

12.2 Finding a Maximizer of the Risk . ... .. .. .. 84

12.3 Connection to the Kalman Filter and E-Minimax Approach . . 87

12.4 Numerical Example . .. .... .. 90

13 STATE ESTIMATION WITH INITIAL STATE UNCERTAINTY .. . 96

13.1 Conditional Mean Estimators ....... .. .. 97

13.2 Approximations to Minimax Estimators ..... .... .. 98

13.3 Numerical Example ... . ..... .. .00

14 CONCLUSIONS ......... . ... .. 106

APPENDIX

A PROOF OFLEMMA 5.2 . . . 1..07

B PROOFOFLEMMA9.1 ............. ...........109

C PROOF OFLEMMA 9.2 . . .. .... .10

REFERENCES . .. . ... .111

BIOGRAPHICAL SKETCH ......... ... .. .. 114

LIST OF FIGURES

Figure page

12-1 Achieved Bayes risk vs. maximum risk . .... .. .. 94

12-2 The maximum risk over O, of the Bayes, minimax, and restricted risk Bayes solu-

tionsvs.e ............. ............... 95

13-1 A full view of 31o', which is a finite (6, V)-dense subset of 31o .. .. .. .. .. .. 103

13-2 A zoom-in view of the bottom left comer of 31o', which is a finite (6, V)-dense subset

of31o. ........ ... ......... .......104

13-3 The risk of .i-(To), the derived -,-optimal estimator, as a function of .ro E 31o' .. .. 104

13-4 The a priori distribution -ro, which is defined on 31o' .. .. .. .. .. 105

13-5 The maximum risk of the Kalman Filter initialized with zero mean and covariance

O.2I as a function of o . ..... .. 105

LIST OF ABBREVIATIONS

CM: conditional mean ......... . ... .. 97

KF: Kalman Filter ......... . .. .. 11

LMMSE: linear minimum mean squared error . ... .. 11

MSE: mean squared error ......... . . .. 45

Abstract of Dissertation Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Doctor of Philosophy

STATE ESTIMATION: A DECISION THEORETIC APPROACH

By

Yoav N. Levinbook

August 2007

Chair: Tan F. Wong

Major: Electrical and Computer Engineering

The problem of state estimation with stochastic and deterministic (set membership)

uncertainties in the initial state, model noise, and measurement noise is approached from

a statistical decision theory point of view. The problem is initially treated within a general

framework in which the state estimation problem is a special case. General existence results such

as the existence of a minimax estimator and a least favorable a priori distribution are derived

for the state estimation problem. Then, attention is restricted to two important cases of the state

estimation problem. In the first case uncertainties in the initial state, model noise, and observation

noise are considered. It is assumed that the a priori distributions of the initial state and the noises

are not perfectly known, but that some a priori information may be available. The restricted

risk Bayes approach, which incorporates the available a priori information, is adopted. When

attention is restricted to affine estimators based on a quadratic loss function, a systematic method

to derive restricted risk Bayes solutions is proposed. When the filtering problem is considered,

the restricted risk Bayes approach provides us with a robust method to calibrate the Kalman filter,

considering the presence of stochastic uncertainties. This method is illustrated with an example

in which Bayes, minimax, and restricted risk Bayes solutions are derived and their performance

is compared. In the second case only the initial state uncertainty is considered. The initial state is

regarded as deterministic and unknown. It is only known that the initial state vector belongs to a

specified parameter set. The (frequentist) risk is considered as the performance measure and the

minimax approach is adopted. The search of estimators is done within the class of all estimators.

If the parameter set is bounded, a method of finding estimators whose maximum risk is arbitrarily

close to that of a minimax estimator is provided. This method is illustrated with an example in

which an estimator whose maximum risk is at most 3%b larger than that of a minimax estimator is

derived.

CHAPTER 1

INTRODUCTION

The problem of state estimation for linear dynamical systems has received considerable

attention in signal processing, controls, communications, econometrics, and a wealth of other

fields. The usual formulation of the problem assumes that the initial state, model noise, and

measurement noise are random vectors with perfectly known a priori distribution or at least with

known covariance and mean. It is well known that if these assumptions, together with other usual

assumptions regarding the noises, are valid, the Kalman filter (KF) [1] is the linear minimum

mean squared error (LMMSE) estimator. If in addition all the stochastic quantities are Gaussian,

the KF is the minimum mean squared error estimator.

Since the assumption of complete knowledge of the a priori distribution is seldom satisfied,

a Bayesian approach is used in practice. The a priori distributions of the initial state, model

noise, and measurement noise are learned from past experience and used as approximations of

the corresponding true distributions. Nevertheless, even if extensive past experience is available,

the estimated distributions may still deviate from the true ones. The effect of such errors in the

a priori information on the performance of the KF is studied in [2]-[4]. The effect of the errors

in the a priori information of the initial state, model noise, and measurement noise may be very

significant and a KF updated based on erroneous a priori information may perform poorly. Thus

it is necessary to consider other approaches that are robust against uncertainties in the a priori

distribution of the initial state, model noise, and measurement noise.

The state estimation literature deals extensively with the general problem of linear systems

with stochastic or deterministic uncertainties using game theory and the minimax approach (cf.

[5]-[14] and the references therein). Usually the so-called E-minimax approach is adopted. The

F-minimax approach [15] regards the parameter as random with its distribution lies in a class F.

However, the exact distribution in the class is unknown. A F-minimax estimator is an estimator

that minimizes the supremum of the Bayes risk, where the supremum is taken over all elements

of F. When the F-minimax approach is used, the class of available estimators is usually restricted

to linear or affine estimators. As a result, an element of the class 0 is specified by a first-order

statistic (mean) and second-order statistic covariancee) pair.

There are several other approaches in statistical decision theory that seem suitable in this

context. Among the most prominent approaches is the restricted risk Bayes approach. The

restricted risk Bayes approach, proposed by Hodges and Lehmann [16], is a compromise between

the Bayes approach and the minimax approach. A restricted risk Bayes estimator minimizes

the Bayes risk with respect to an a priori distribution suggested based on some past experience

subject to the restriction that the maximum risk does not exceed the minimax risk by more than a

given amount. This approach utilizes available a priori information but at the same time provides

a safeguard in case this information is not accurate. If the a priori information is fairly accurate,

a restricted risk Bayes estimator has good Bayes risk properties. Other work considering the

restricted risk Bayes approach or closely related approaches include [17]-[20]. Despite the

appealing formulation of the restricted risk Bayes approach, it has not been utilized in the context

of state estimation.

Although the problem of state estimation with stochastic uncertainties has been approached

from the F-minimax approach, we believe that approaching this problem from the restricted risk

Bayes approach also has a considerable merit. If a state estimation problem can be regarded as

a zero-sum two-person game (henceforth to be referred as a game) against a rational opponent,

then the F-minimax approach seems very attractive. However, in most applications, if we regard

the state estimation problem as a game, the game is against Nature. Using the F-minimax

approach in this case corresponds to a very pessimistic viewpoint that regards Nature as a rational

opponent that wishes to cause us the largest possible loss. The F-minimax approach may still

seem reasonable in the case that there is no a priori information that enables us to regard certain

distributions in C as more likely than others. However, in many applications, some a priori

information regarding the true distribution may be available. This a priori information may be in

the form of a nominal distribution, which is suggested based on some past experience. It is well

known that under certain conditions, a F-minimax estimator is a KF relative to a least favorable a

priori distribution. If our a priori information suggests that the true distribution is very different

from a least favorable a priori distribution, then using the F-minimax approach may result in

very conservative estimators. The restricted risk Bayes approach enables us to adopt a less

conservative but still rather robust approach, which gives us a guaranteed safeguard in terms of

the risk. We assume the nominal distribution is an approximation of the true a priori distribution

and search for an estimator that minimizes the Bayes risk relative to the nominal distribution

subject to the constraint that the Bayes risk relative to any distribution in 0 is less than a given

value. We can determine this value based on the amount of past experience that we have. The

stronger the available past experience, the more we can trust the approximated distribution and

the larger the value we can allow.

In this work, we consider the restricted risk Bayes approach. Under our formulation, the

F-minimax approach is a special case. We consider the risk, based on quadratic loss functions,

as our performance measure. In this case, we restrict ourselves to affine estimators in order

to derive estimators that are attractive in terms of computational complexity. We provide a

systematic method for solving restricted risk Bayes and E-minimax solutions. In some important

cases, this method can be easily used to calibrate the KF, considering the presence of stochastic

uncertainties.

While in most applications, the observation noise can be indeed modeled as random, or

even Gaussian (e.g., the thermal noise in communications systems), in some applications one

can argue whether the initial state can be better modeled as an unknown constant, rather than as

random, and the same may be also argued for the model noise. In this work, we also consider the

case that there is a deterministic uncertainty in the initial state as a special case of the problem

of state estimation with deterministic uncertainties. The distributions of the model noise and

measurement noise are assumed known. As mentioned above, it may be reasonable to assume

the model noise is deterministic and unknown as well. We do not pursue this approach here

since we believe that this would obscure the results that we derive for the initial state uncertainty

due to the technical difficulties of considering deterministic uncertainties in the model noise.

In addition, if we are concerned with estimation problems in which it is necessary to estimate

fast, using a small number of samples, the uncertainty in the initial state may a have a much

more significant effect than the uncertainty in the model noise. The reason is that if the estimated

signal changes slowly (which is the case in many applications), the uncertainty in the model

noise may be small whereas the uncertainty in the initial state may be very large. For instance,

in tracking problems the changes of the velocity or acceleration of the target between adjacent

samples are usually small relative to the range of their possible values at the initial time. When

deterministic uncertainty is considered in the initial state, the minimax approach may be the

preferable approach. A minimax estimator minimizes the supremum of the (frequentist) risk.

The minimax approach seems a reasonable approach when it is only known that the initial

state vector belongs to a certain parameter set. The minimax approach is especially suitable for

applications in which we may interpret the estimation problem as a game against an opponent.

This is certainly the case in many military applications.

The state estimation literature has traditionally focused on the Bayesian approach and the

F-minimax approach when dealing with uncertainty in the initial state vector. The classical

approach that regards the initial state vector as deterministic, has been mostly ignored. This is

despite the fact that in many important applications (see for example [21] and [22]), it seems

more reasonable to model the initial state vector as deterministic and unknown. One example that

falls within the classical estimation framework is Danyang and Xuanhuang [23], where the state

estimation problem was considered from a least squares viewpoint and the best linear unbiased

estimator was derived. In [24], the authors consider the problem of state estimation with initial

state uncertainty from a decision theoretic point of view. The initial state vector is regarded as

deterministic and unknown. It is only known that the initial state vector belongs to a parameter

set. The risk, based on quadratic loss functions, is considered as the performance measure. The

search for a minimax estimator is done within the class of all possible estimators. Minimax

estimators are derived for the case of unbounded parameter set and approximations of minimax

estimators are derived for the case of bounded parameter set. In this work, we will repeat some

of the results in [24] that deal with the bounded parameter set case in order to illustrate how

minimax estimators can be approximated with arbitrarily prescribed accuracy.

While we are mainly interested in the state estimation problem, a large part of this work will

be concerned with a more general estimation problem. In fact, some of the existence results such

as the existence of a minimax estimator and a restricted risk Bayes solution hold in a very general

setting and may have applicability not only in the state estimation problem.

The rest of this work is organized as follows. In Chapter 2, we present notation and

conventions that are used throughout this work. In Chapter 3, we present a general decision

theoretic formulation that is needed in order to address the problem of state estimation with

stochastic and deterministic uncertainties and also to derive other, more general, results. In

Chapter 4, we derive several general decision theoretic results, which are based on well known

results from decision theory and game theory; the applicability of these results is not limited

only to the problem of state estimation with stochastic and deterministic uncertainties. In

Chapter 5, we consider the case that the risk function is specified by a loss function. We derive

rather weak conditions that guarantee the existence of a minimax estimator and a restricted risk

Bayes solution. In Chapter 6, we restrict ourselves to convex loss functions, in general, and the

quadratic loss function, in particular. In Chapter 7, we discuss how a minimax estimator can

be found by solving the dual problem of finding a least favorable a priori distribution. Since,

in general, finding a minimax estimator may be an extremely difficult task, in Chapter 8 we

discuss how one can derive approximations to minimax estimators, where the approximation

can be made as accurate as desired. In Chapter 9, we consider the general restricted risk Bayes

estimation problem and show that this problem is equivalent to a sequence of minimax estimation

problems. In some estimation problems there are restrictions on the observations that can be

used in order to estimate the parameters. This is the case in the state estimation problem in which

each state can be estimated using only certain observations. We consider this type of estimation

problems in Chapter 10. In Chapters 11, 12, and 13, we restrict ourselves to the problem of state

estimation with uncertainties in the initial state, model noise, and measurement noise, which

is the main problem considered in this work. In Chapter 11, we derive some general existence

results that are based on the results of the previous chapters. In Chapter 12, we consider the case

of stochastic uncertainties in the initial state, model noise, and measurement noise, and restrict

ourselves to affine estimators. We propose a method that can be easily used to derive a restricted

risk Bayes solution in many important cases. In Chapter 13, we consider the case of deterministic

initial state uncertainty, and search for estimators within the class of all estimators. We conclude

this work in Chapter 14.

CHAPTER 2

GENERAL NOTATION AND CONVENTIONS

Let R"N denote the N~-dimensional Euclidean space, and let RW = RIW. Let R"~ = {a e

R"N : a(i) > 0 for i = 1, 2, ..., N}. We use RWNxM to denote the space of NV-by-M~ real

matrices. Let S" denote the space of n-by-n real symmetric matrices. Let S" denote the cone

of positive semi-definite matrices in S". Let N = {0, 1, .. .}. Let a denote an NV dimensional

vector. We use a(i) to denote the ith element of a and |a| to denote the Euclidean norm of a.

We let ||a|| = CE |a(i)|. Let Ai and B denote arbitrary matrices. We use N~(A) to denote

the nullspace of A, At to denote the Moore-Penrose pseudo inverse of A, tr(A) to denote the

trace of A, and ||IA||2 to denote the 2-norm of A. We use A > 0 (A > 0) to denote that A

is positive definite (positive semi-definite) and symmetric and A > B (A > B) to denote

that A B > 0 (A B > 0). We use A 0 B to denote the Kronecker product of A and

B. Let A denote an arbitrary set. We use | A| to denote the cardinality of A. If AC R ", we

let A = { x E R"N : x~y = 0, for ally E A}.Let A1,. ,AS be arbitrary sets. Let

A = n0 Ali, where product on sets means cartesian product. We use vri : A Ai to denote

the projection onto the ith factor. Given a s A, let a(i) = xri(a). If Al,. ,AS are subsets of

vector spaces V1, .. respectively, a' and a" are in A, and as R I, then a = a' + a" means

that a(i) = a'(i) + a"(i) for i = 1,. ., n, and a = caa' means that a(i) = caa'(i) for i = 1, .., n.

Let f : X Y and let Z be a subset of X. Then f (Z) = {f (z) : z E Z}. Let lni denote the

NVx NV identity matrix. Let ONxM/ denote the NVx M~ zero matrix. When the dimensions of the

zero matrix and identity matrix are clear from the context, they will simply be denoted 0 and I,

respectively. For any topological space X, we use B(X) to denote the o--algebra of Borel subsets

of X. Given a measurable space (X, FT), where X is a subset of R"N and FT = B(X), and a

probability measure -r defined on (X, FT), by the mean vector and autocorrelation matrix of -r, we

mean the mean vector and autocorrelation matrix of a random vector that is distributed according

to -r, i.e., the mean vector of -r is fx xd-r(x) and the autocorrelation matrix of -r is fx xx~d-r(x).

Let S denote an index set, possibly uncountable, and (Xs, F,), for each s e S, be a measurable

space. Then nses Fs, denotes the o--algebra on nses X, generated by subsets of the form

nses A,, where A, E T, for each s ES and A, = X, for all but a finite number of s E S. In

particular, given measurable spaces (X1, Fi), (X2, F2,),. ., (XN, Fy), FI~x T~ 2 x FTN

denotes the o--algebra on X1 x X2 x x XN generated by subsets of the form Al x A2 x x AN,

where Ai E Fei for i = 1,. ., NV. Given a measurable space (X, FT), let m(FT) denote the set of all

real-valued measurable functions on (X, FT) and m(FT) denote the set of all extended real-valued

measurable functions on (X, FT). Given real numbers a and b, a Vb denotes max {a, b} and a Ab

denotes mina, b}. Some of the derivations in the sequel require the use of the extended real

number system. We use the usual conventions for arithmetic operations in the extended reals.

When we take the supremum or infimum of a subset AC R we regard A as a subset of the

extended reals, i.e., the supremum and infimum always exist and may take the values +oo and

-oo, respectively.

CHAPTER 3

DECISION THEORETIC FORMULATION

Let (R, &~, Q) be a probability space, where R is the sample space of an experiment and

elements of -TA are the events of the experiment. Suppose there are given a set 8, called the set of

possible states of nature, and a family P = { Po : 0 E 8}) of probability measures defined on the

measurable space (R, &~). The probability measure Q is unknown to the experimenter; however

it is known that Q is an element of P. Suppose there is given a measurable space (Y, FTy). The

experimenter observes the value y taken by a random element (or a Y-valued random variable )

Y : R (f FTy); this value is called an observation.

In Wald's interpretation [25] of the statistical decision problem as a zero sum two-person

game, Nature chooses an element 8 E 8, called the true state of nature. The experimenter needs

to reach a certain decision based on the observation y without knowing the true state of nature.

The experimenter reaches such a decision by choosing a decision rule from a class of decision

rules. Here we are mainly interested in estimation problems, although some of the results of this

work apply also to decision problems.

In classical estimation, the experimenter estimates the true state of nature 8 E 8 or more

generally a function of 0. In this case the space (R, -T2, Q) can be taken to be (Y, FTy, Q), where

Q is the distribution of Y. In this work, we are interested in a more general case. Suppose

there is given an additional measurable space (X, 6T) and a random element X : R

(X, 8Tx). Suppose Y and X have a joint distribution defined on the product measurable space

(y x X, FTy x 5T). Suppose the experimenter wants to estimate the value x taken by X. In this

case, the space (R, -T2, Q) can be taken to be the space (y x X, FTy x 8x, Q), where Q is the

joint distribution of Y and X and is an element of P. Note that the previous mentioned case can

be regarded as a special case, in which the distribution of X assigns mass 1 to the true state of

nature 8 E 8.

Suppose the experimenter is allowed to choose estimates for x from a class D. The class

D, equipped with a topology, is referred to as the space of possible estimates. Often D coincides

with X. However, it is convenient not to make this restriction. Given a topological space A, let

My/1 denote the class of all probability measures on (A, B(A)). A nonrandomized estimator i-

is simply a ~T-measurable mapping from Y into D. A mapping :i : Y M zD, y H 9(I)

is said to be a randomized estimator if y H -(D'| y) is a 8 y-measurable function for all

D' E B(D). A randomized estimator (from now on just an estimator) can be used to determine

uniquely a procedure for choosing an estimate for :r. The estimate of :r, given y is observed

and i- is used, is an element of D selected according to the probability measure i-(-|y). A

nonrandomized estimator i- can be regarded as a randomized estimator that assigns to each

ye E a Dirac measure, i.e., a measure that assigns a mass 1 to a single point in D. Let 2'

denote a class of estimators for :r. Since we adopt a decision theoretic approach, the merit of

an estimator will be judged based on a risk function. The risk function is the expected loss

incurred to the experimenter when using an estimator :i and 8 is the true state of Nature. Let

R : 8 x X R WU {+oo} denote a risk function. In the formulation of this work, an estimation

problem may be specified by a triplet (8, 2', R).

For the convenience of the reader, we present here some basic decision-theoretic definitions

taken from [26] and [25], with slight modifications. These definitions are given for an estimation

problem (8, 2 R).

Definition 1. An estimator :i* E 2' is said to be a nzinintax estimator if

sup R(8, i-*) inf sup R(0,;i).

Definition 2. Let e > 0. An estimator :i* E X is said to be an e-nzinintax estimator if

sup R(8, i-*) < inf sup R(8, i-) +

BOe *E~ aee

Definition 3. An estintator i-' is said to be as good as an estintator i-" if R(0, i-') < R(0, i-") for

all 8 E 8. An estintator i-' is said to be better than an estintator i-" if R(0,;i') < R(0,;i") for all

8 E 8 and R(0, i-') < R(0, i-") for at least one 8 E 8. An estintator i-' is said to be equivalent to

an estintator i-" if R(0, i-') = R(0, i-") for all 8 E 8.

Definition 4. An estimator & is said to be admissible if there exists no estimator better than 2. An

estimator is said to be inadmissible if it is not admissible.

Definition 5. A class A of estimators is said to be essentially complete relative to a class B of

estimators if given any estimator & in B, there exists an estimator i* E A that is as good as 2.

If AC c is essentially complete relative to X, we simply say that A is essentially

complete.

Definition 6. An estimator & is said to be an essentially unique minimax estimator if any

minimax estimator is equivalent to 2.

Let V(8, X, R) = infecx supeoe R(0, 2) be the minimax risk. The following assumptions

will be assumed throughout this work, unless mentioned otherwise:

Assumption 3.1. V(8, K, R) < +oo.

The case V(8, .[, R) = +oo is not very interesting. Clearly in this case the minimax

approach is not suitable since any estimator is a minimax estimator.

Assumption 3.2. 8 is a metric space with metric a.

Assumption 3.3. The risk function R is nonnegative.

Assumption 3.3 can be weakened to the assumption that infeee int ,x R(0, 2) > -oo, but

for the sake of presentation it is advantageous to assume that R is nonnegative.

We need additional definitions. If & is such that R(-, 2) is bounded from below and is a

B(8)-measurable function, we denote the Bayes risk relative to -r Me Zl of &

~(~,i (3J8

In this work, we consider only estimation problems (8, X, R) for which R(-, 2) is bounded from

below and is a B(8)-measurable function for all ie E Hence the Bayes risk is always defined.

Definition 7. An estimator i* E X is a Bayes solution relative to -r Me Zl ifr (-, i*)=

infEx gr (-, 2). An estimator i* E X is a Bayes solution if it is a Bayes solution relative to some

distribution -r Ms/l.

Definition 8. An estimator & is said to be an essentially unique Bayes solution relative to -r if any

Bayes solution relative to -r is equivalent to x.

Note that we may regard a point 8 E 8 as an element of Me/l by regarding it as the

probability measure that assigns mass 1 to 0. Thus 8 may be considered as a subset of Mel. With

this viewpoint, r(-, 2) is an extension of R(-, ) from 8 to Me/l, i.e., r(0, 2) = R(0, 2) for all

S8.O

We also use the notion of Bayes solution in the wide sense:

Definition 9. Let {n }r be a sequence of a priori distributions in Me/l and i* be an estimator

Then i* is a Bayes solution relative to the sequence {n }) if

lim [r(ni, i*) -- inf r(i, 2)] = 0.

An estimator i* is a Bayes solution in the wide sense if there exists a sequence {nri } EZ/e such

that i* is a Bayes solution relative to the sequence {8 }).

It iseasy to show that for each re M 2e, inf ~xr-,~: I ~,2 V(8, K, R). Thus by

Assumption 3.1, infe,x r (-, 2) < +oo, and the term r (-r, i*) infa,x r (-r, 2), in the above

definition, is well defined.

Definition 10. An a priori distribution -ro E Me/l is said to be least favorable if

1111T T, Z Sup inf r 7r, x.

Definition 11. The estimation problem (8, I R) is said to be strictly determined if

inf sup r(-r,i)= sup inf r(-r, ).

For any class AC c of estimators, we let A(M~, R) = { E A : supose R(0, 2) < M}).

Definition 12. Let -r Me Zl and Co ERIWU {+oo }. An estimator io is said to be a restricted risk

Bayes solution relative to (-r, Co) if

T T,0 111 T T I

Note that if Qo = V(8, K, R), then for any -r Me Zl, a restricted risk Bayes solution

relative to (-r, Qo) is a minimax estimator. In addition, a restricted risk Bayes solution relative to

(-r, +oo) is a Bayes solution relative to -r. Thus the problems of finding a minimax estimator and

a Bayes solution may be regarded as two extreme cases of the problem of finding a restricted risk

Bayes solution.

CHAPTER 4

GENERAL DECISION THEORETIC RESULTS

In this section, we present several decision theoretic results for an estimation problem

(8, X2, R). These results hold in a much more general case than the estimation problem

considered in this work and may be used in general decision theoretic problems. These results are

based on well known results from decision theory and game theory.

First, we note that it is well known that (cf. [26, Exercise 2.2.1])

sup R (B, i) sup r (-r, .). (4-1)

Consider the space Me/l equipped with the topology of weak convergence [27, pp. 236]. The

topology of weak convergence makes Me/l a Hausdorff space.

Lemma 4.1. Suppose 8 is compact. Then

1. Me/l is nzetrizable and compact.

2. Let~i e E be such that R(0,.i-) < +oc for each Ie 8 and R(-,.i) is continuous on 8.

Then r (-, i) is continuous on M sl.

3. Let~i e E X be such that R(0,.i-) < +oc for each Ie 8 and R(-,.i) is upper senicontinu-

ous on 8. Then r (-, i) is upper senticontinuous on M sl.

Proof 1) By the hypothesis of the lemma, 8 is a compact metric space and hence is separable

(cf. [28, Exercise 2.25]). This implies that Me/l is metrizable [29, pp. 122]. In addition, [29,

Theorem 3.1.9] furnishes that Me/l is compact.

2) By [29, Theorem 3.1.5], J, fdvr is a continuous function of -r for any bounded and

continuous f. Since R(-,.i) is a continuous real-valued function on a compact set, it is bounded.

Hence r(-,.i) is continuous on Ms/l.

3) By [29, Theorem 3.1.5], J, fdvr is an upper semicontinuous function of -r for any up-

per semicontinuous f that is bounded from above. Since R(-,.i) is an upper semicontinuous

real-valued function on a compact set, it is bounded from above. Hence r(-,.i) is upper semicon-

tinuous on Me/l for any .i E C. O

Given a set Z, let & (Z) denote the space of nonnegative extended real-valued functions on

Z equipped with the topology of pointwise convergence. For u', u" E W (Z), we write u' < u"

to denote that u'(z) < u"(z) for all z E Z. Consider the space W (8). Let A denote an arbitrary

set and f : 8 x A [0, +oo] an arbitrary function. Then for each a s A, f (-, a) E W (8). Let

A' and A" denote arbitrary sets, and let f' : 8 x A' [0, +oo] and f" : 8 x A" [0, +oo]

denote arbitrary functions. It is convenient to use the notation (A', f') -4 (A", f") to denote that

for each element a" E A", there exists an element a' E ~A' such that f'(-, a') < f"(-, a"). Let

(A', f') ~ (A", f") denote that (~A', f') -4 (A", f") and (A", f") -4 (~A', f'). Clearly ~ is an

equivalence relation. Now, consider an estimation problem (8, .[, R), and let A' and A" denote

classes of estimators (i.e., subsets of X). In this case, (A', R) -4 (A", R) simply states that the

class A' is essentially complete relative to ~A". In this case, the notation can be simplified, and we

can write A' -4 A" to denote that A' is essentially complete relative to A". Note that the relation

-4 is a preorder on the collection of all classes of estimators. Clearly A -4 A, and it can be easily

verified that A -4 B and B -4 C imply A -4 C. Similarly, if A and C are classes of estimators, we

use A ~ C instead of (A, R) ~ (C, R).

Definition 13. A subset U of W (Z) is said to be half-closed if for each a in the closure of &,

there exists a u* E U such that u* < u.

Note that given an estimation problem (8, X, R), for each is E r (-, ) E W (Mel) and

R(-, 2) E & (8). Let M(A, Me/l) = {r(-, 2) : S E A} and M(A, 8) = {R(-, ) : f E A}. The

following definition is due to LeCam [30].

Definition 14. Given an estimation problem (8, K, R), a subset A of K is said to have the

property (W) if& (A, 8) is half-closed.

We need also the following closely related definition.

Definition 15. Given an estimation problem (8, K, R), a subset A of K is said to have the

property (W*) if&(A,Me/l) is half-closed.

The preceding definition can be reformulated as follows: Given an estimation problem

(8, K, R), a subset A of .[ is said to have the property (W*) if for each net {idaeA n A,

there exists a subnet {:ib bEB and an element i-* E A such that lim inf r(-, i'b) > r( ~).

Indeed, suppose #'(A, Me/l) is half-closed. Let {:i'o,,A denote a net in A. Let U denote the set

of limit points of this net. The set U is not empty since the space ~(Mel) is compact [31]. Since

~(Mel) is compact, M(A, Me/l) is relatively compact. Thus there exists a subnet {ib bEB that

converges to a point it in the closure of d(A, Me/l). Hence lim inf R(-, ib) = lim R( *,;b) = IL.

Since '(A, Me/l) is half-closed, there exists an element t' E M(A, Me/l) such that < Is.

Hence there exists an element i-* E X such that R(-,;i*) < liminf R(-,;ib). Suppose

for each net {:i )aEA in A, there exists a subnet {:ib bEB and an element i-* E A such that

lim inf r (-, ib) > r (-,;i*). Let it* belong to the closure of s'(A, Me/l). Then there exists a net

{u,),eA Ed 8(,iZ) that converges to u*. Clearly for each element u, in this net, there is

an element i's E A such that u, = R(-,;i). Thus lim R(-, is) = u*. It follows that there

exists a subnet {:ib bEB and an element i-* E A such that lim inf R(-, i'b) > R(-,;i*), whence

u* > R(-,;i*) and M(A, Me/l) is half-closed. In entirely analogous way, A has the property (W)

if and only if for each net {:i' )aEA in A, there exists a subnet {:ib bEB and an element i-* E A

such that lim inf R(-, ib) > R(-,;i*). Since each 8 can be identified as an element in Me/l, as

discussed previously, it is clear that the property (W*) implies the property (W). The property

(W*), as formulated with nets, is closely related to Wald's weak compactness [25, pp. 53].

The only difference is that in the current definition sequences are replaced by nets. Hence the

property (W*) is weaker than Wald's weak compactness. Sufficient conditions for a subset A of

X to have the property (W) are given in [31]. A simple sufficient condition is that there exists

a Hausdorff topology J for A such that A is compact and R(0, -) is lower semicontinuous on A

for each 8 E 8. Similarly, a sufficient condition for A to have the property (W*) is that there

exists a Hausdorff topology J for A such that A is compact and r(-r, -) is lower semicontinuous

on A for each -r Me Zl. Suppose there exists a topology J for A such that A is compact. Then

if the topological space (A, J) satisfies the first axiom of countability, by Fatou's Lemma, the

requirement that r(-r, -) is a lower semicontinuous on A for each -r Me Zl can be replaced by the

requirement that R(0, -) is a lower semicontinuous on A for each 8 E 8.

The following definition appears in [31].

Definition 16. A class A of estimators is said to be subconvex if for any i', i" E A and

0 < a~ < 1, there exists E A such that caR(0, i') + (1 ~) R(0, i") > R(0, 2) for all 8 E 8.

Clearly if A is a convex subset of a certain vector space and R(0, -) is convex over A for

each 8 E 8, then A is subconvex.

We are now ready to state several theorems some of which will be used throughout this work

and some of which are presented for their own sake.

Theorem 4.1. Given an estimation problem (8, X, R). Suppose that K has the property (W).

Then there exists a minimax estimator

Proof The proof is very similar to the proof of Wald's Theorem 3.7 [25]. The main differences

is that sequences are replaced by nets. Recall that we assume that V(8, X9, R) < +oo. Let {x,}

be a sequence such that supose R(8, in) converges to infc,x supose R(0, 2). Then there exists a

subnet {idaeA and an element i* E X such that lim inf R(-, is) > R(-, i*). Since {idaeA iS a

subnet of {x,}, lim sup,, R(-, i) > lim inf R(-, fe). In addition, lim,,, supose R(8, in)

lim sup,, R(0, in) for all 8 E 8. Thus in f a supose R(0, i) > supose R(0, i*), and i* is a

minimax estimator. O

After addressing the existence of a minimax estimator, we want to address the existence of a

restricted risk Bayes solution.

Theorem 4.2. Given an estimation problem (8, X9, R). Suppose .9 has the property (W*).

Then if V(8, K, R) < Qo, there exists a restricted risk Bayes solution relative to (v, Qo).

Proof Suppose V(8, X9, R) < Q. Note that the class X9(Qo, R) is not empty even if

Qo = V(8, K, R) since by Theorem 4.1i, there exists a minimax estimator.

Let {i,} be a sequence in X(Qo, R) such that limes, r(v, 2,) = infiEX(co,R) doI, 2).

Then since .9 has the property (W*), there exists a subnet {idaeA Of (Xn} and an element

i* E K such that liminf r(-r, i) > r(-r, *) for each r E Mel. Certainly r(-r, ,) < Qofor

all n and each -r E Mo/. Since {idaeA is a subnet of {x,}, it is clear that lim inf r(-r, 2,) < Co

for each -r Me Zl, whence R(0, i*) < Co for each 8 E 8. Thus i* E X(Qo, R). It is also

clear that infier(co,,) r(v, 2) > r(v, i*). Thus i* is a restricted risk Bayes solution relative to

Putting Qo = +oo in the above theorem, we get that for each -r Me Zl, there exists a Bayes

solution relative to -r (provided that X has the property (W*)).

The following Theorem, a version of Wald's complete class theorem, appears in [31].

Theorem 4.3. Suppose X is subconvex and has the property (W). Then the class of Bayes

solutions in the wide sense is essentially complete.

The following theorem is essentially a corollary to a very well known game-theoretic result

[32, Theorem 4.2].

Theorem 4.4. Suppose 8 is compact, K is subconvex, R(-, 2) < +oo for each & E K, and

R(-, 2) is upper semicontinuous on 8 for each is K Then the estimation problem (8, X, R)

is strictly determined and there exists a least favorable a priori distribution.

Proof Fix is E Since R(-, 2) is an upper semicontinuous real-valued function on a compact

set 8, it is bounded. Thus r (-, ) is bounded on 8, and hence a real-valued function. By Lemma

4.1, Me/l is compact and r(-, 2) is upper semicontinuous on Me/l for each is E It is easy to

verify that since X is subconvex, for each i', i" E X and 0 < a~ < 1, there exists i s such

that r (-, 2) < ~r (-r, i') + (1 a~)r (-, ") for all 8 E 8. Certainly r (-, ) is concave on Me/l for

all ie E Applying [32, Theorem 4.2],

1111 Sup r 7r, x sup inf r 7r, x.

Thus (8, X, R) is strictly determined. Since inf~Ex r(-, 2) is upper semicontinuous on a

compact set Me/l, there exists -ro E Me/l such that

III T TOZ) Sup inf r (r, x).

Thus -ro is a least favorable a priori distribution. O

The assumption in Theorem 4.4 that R(-, 2) < +oo for each i s and R(-, 2) is upper

semicontinuous on 8 for each i s is obviously too strong. In the following theorem this

assumption is considerably weakened, but at the price of a stronger assumption on X. Let

& C K denote the class of Bayes solutions relative to -r Me Zl. Before stating the theorem, we

need the following lemma.

Lemma 4.2. Suppose .F has the property (W*), then

infr~~i =inf r(-r, i) for all y E Me-lo

Proof In Theorem 4.2 it is shown that if X has the property (W*), there exists a Bayes solution

relative to -r for all -r Me Zl. The lemma follows easily. O

Theorem 4.5. Suppose 8 is compact, K is subconvex and has the property (W*), R(-, 2) <

+oo for each is E and R(-, ) is upper semicontinuous on 8 for each is E Then the

estimation problem (8, X, R) is strictly determined and there exists a least favorable a priori

distribution.

Proof Let G and Go denote the convex hull of (.[, Me/l) and &(9, Me/l), respectively. It

is easy to see that each g EG is a concave upper semicontinuous real-valued function on the

compact set Me/l. Thus by [32, Theorem 4.2],

inf sup g(-r) =sup inf g(-r). (4-2)

Fix -r Me Zl. On one hand, since &(9, Me/l) C Ga,

inf g(-r) < inf gr)=if(, )

On the other hand, if we fix g E G'a, then there exist an integer n > 0, real numbers

ai1, an > 0, and elements ul, ., an e (9, Mel) such that g = CE asse and

I~ as = 1. Let 1 < i < n be such that a () < Up (7) for j = 1,. ., n. Then g(-r) > a ().

Thus

inf g(-r) > inf gr)

It follows that

inf g(r) = inf r (-, 2). (4-3)

Since G > Ga,

inf sup g (r) < inf sup g (-). (4-4)

g6G 76MenL g6GSB 76Men

Fix g E G. Then since X is subconvex, there exists i s such that r (r, i) < g(-r) for all

-r MeiZ Hence

inf sup r (-, ) < inf sup g (-). (4-5)

26X 76enL g6G 76MenL

By (4-2)-(4-5) and Lemma 4.2, (8, X, R) is strictly determined. Since info,EG g is upper

semicontinuous on the compact set Me/l, there exists -ro E Me/l such that

inf g(-ro) = sup inf g(-r).

g6GSB 76Men g6GgB

By (4-3) and Lemma 4.2, -ro is a least favorable a priori distribution. O

If the compactness of 8 is dropped in the above theorem, then there may not be a least

favorable a priori distribution. If there exists no least favorable a priori distribution, but the

estimation problem (8, X, R) is strictly determined, a minimax estimator can be found as a

Bayes solution relative to a least favorable sequence of a priori distributions, i.e., a sequence

{7r,} E Me/ that satisfies lim,,, infie g r(-r,, i) = supe~Me inf~Ec r(r, i). We are not going

to deal with the question of how such a sequence can be constructed. The following theorem is

essentially Wald's Theorem 3.9 in [25].

Theorem 4.6. Suppose the estimation problem (8, X, R) is strictly determined. Then if -r is a

least favorable a priori distribution, any minimax estimator is also a Bayes solution relative to

Proof It can be verified that the proof of [25, Theorem 3.9] applies without any changes. O

When 8 is compact, we have the following version to Wald's complete class theorem.

Theorem 4.7. Suppose 8 is compact, K is subconvex and has the property (W*), R(-, 2) is

lower semicontinuous on 8 for each is K R(-, 2) < +oo for each is E and R(-, ) is

continuous on 8 for each is E Then & is essentially complete.

Proof Since the class of Bayes solutions in the wide sense is essentially complete, we are done

if we show that any Bayes solution in the wide sense is a Bayes solution. Suppose that i* is a

Bayes solution in the wide sense. Then there exists a sequence {n}i E Me/l such that

lim [ n n 2 *)] = 0 (4-6)

By Lemma 4. 1 part 1), there exists -rs Me Z and a subsequence {4}i ) Me Zl such that -ri

converges weakly to -r. By the hypothesis of the theorem, r(-, 2) is continuous on Me/l for any

is E Thus infies r(-, 2) is upper semicontinuous on Me/l [33, Proposition 1.5.12]. By Lemma

4.2, ini~,x r(-,2) is upper semicontinuous on Me/l. Since r(-,i*) is lower semicontinuous on

M el, inf a r (-, ) r (-, *) is upper semicontinuous on Me/l and

lim sup[ inf r(i,,) Tirq,,i*)] < inf r(O,i) -- r(q0, *). (4-7)

By (4-6) and (4-7), inf~Ex r(-r, 2) > r(-r, i*), whence i* is a Bayes solution relative to Ire. O

Remark 4.1. Suppose C7 is a metric space and 8 is a subset of W. Let 8 denote the closure

of 8. Then if R(-, 2) is lower semicontinuous on 8 for each is K a minimax estimator for

(8, K R) is also a minimax estimator for (8, X R). In addition, if A is essentially complete

in the estimation problem (8, X, R), it is also essentially complete in the estimation problem

(8, K, R). Thus we can solve (8, X, R) instead of (8, X. R). Indeed, suppose A is essentially

complete in the estimation problem (8, X, R). Fix is E Then there exists i' E A such that

R(0, i') < R(0, 2) for all 8 E 8. Since 8 C 8, R(0, i') < R(0, 2) for all 8 E 8 and A is also

essentially complete in the estimation problem (8, X, R). Fix is E It is easy to show that

the lower semicontinuity of R(-, 2) on 8 implies that supeoe R(0, i) = supose R(0, 2). Since

& is arbitrary, infe, g supose R8, i) = infie g supose R(0, 2). Thus a minimax estimator for

(8, K R) is also a minimax estimator for (8, X R). In particular if C7 is a finite-dimensional

normed space and 8 is a bounded subset of 7, then, II idustI~, loss of generality, we may assume

that 8 is compact since if thri\ is not the case, we can consider the estimation problem (8, X, R)

instead.

In this work it will be sufficient to impose the following conditions:

Condition 4.1. X is subconvex.

Condition 4.2. There exists a compact metrizable space 9 and a function R* : 8 x X*

[ 0, +oo] such that (X9, R) ~ ( 9 ", R*), R* (0, -) is lower semicontinuous on 9 for each 8 E 8,

and R*(-, a) E m(B(8)) for each as K *.

Condition 4.3. R(-, 2) is lower semicontinuous on 8 for each is K R(-, 2) < +oo for each

is 9 and R(-, ) is continuous on 8 for each a E .

Condition 4.4. If~ E X is a Bayes solution relative to -r M Z/e, & is an essentially unique

Bayes solution relative to -r.

In the following corollary, we summarize all the results of this chapter that are needed in the

subsequent chapters.

Corollary 4.1. Suppose Conditions 4.1 and 4.2 hold. Then X has the property (W*), there

exists a minimax estimator and there exists a restricted risk Bayes solution relative to (-r, Co) for

each -r M Z/e and Co > V(8, X9, R). If in addition, 8 is compact and Condition 4.3 holds,

(8, K, R) is strictly determined, there exists a least favorable a priori distribution -ro E Me/l,

any minimax estimator is a Bayes solution relative to -ro, and the class of Bayes solutions is

essentially complete. If in addition, Condition 4.4 holds, then the (essentially unique) Bayes

solution relative to a least favorable a priori distribution -rs Me Z is an essentially unique

nzinintax estimator and is admissible.

Proof Although, the results of this chapter were formulated for an estimation problem

(8, 2', R), there is no use whatsoever of the fact that .9 is a class of estimators. Therefore,

the results also hold for the triplet (8, 2'*, R*), which is, in fact, a zero sum two- person game

(cf. [25]). For the sake of simplicity, we assume that (8, 9 ', R*) is an estimation problem. The

proof can be trivially modified to the case that 9 is not a class of estimators, but an arbitrary

set. Let r*(-r, a) = je R*(0, a)d-r, where -r Me Zl and as E *. It can be verified that since

2'* is a compact metrizable space and R*(0, -) is lower semicontinuous on 2 for each 8 E 8,

{r*(-, a) : as E X} is half-closed, i.e., 2'* has the property (IT*). It can be verified that

since ( 2, R) ~ ( 2*, R*), .9 has the property (IT*). By Theorem 4.1, there exists a minimax

estimator. By Theorem 4.2, there exists a restricted risk Bayes solution relative to (-r, Qo) for

each -r M Z/e and Qo > V(8, 2', R). If, in addition, Condition 4.3 holds, it follows from the

preceding results, that (8, 2', R) is strictly determined, there exists a least favorable a priori

distribution -roE Me/l, any minimax estimator is a Bayes solution relative to -r, and the class of

Bayes solutions is essentially complete. Suppose Condition 4.4 holds as well. Let -r denote a

least favorable a priori distribution. Since any minimax estimator is a Bayes solution relative to

I-r (Theorem 4.6), the (essentially unique) Bayes solution relative to -r is an essentially unique

minimax estimator. By [26, Theorem 2.3.1], this Bayes solution is admissible. O

While sometimes it is possible to verify Conditions 4.1-4.4 directly, other times, especially

when there is no closed from expression for R, it may be difficult to check whether these

conditions hold. In the next chapter, we consider the case that R is based on a loss function L.

In this case, it is possible to derive conditions that can be more easily checked when there is no

closed form expression for R.

CHAPTER 5

THE CASE THAT THE RISK IS SPECIFIED BY A LOSS FUNCTION

A risk function R is usually chosen by first specifying a loss function. A loss function

L : E x D R WU {+oo} specifies that L(:r, d) is the loss incurred to the experimenter

when using the estimate d and :r is the true value of the parameter. Let 9 denote the class of all

(randomized) estimators. Ignoring measurability considerations for now, let L (:r, i) denote the

mapping y H D L(:r, d)d~i(dly). The risk function R : 8 x 9 R WU {+oo} is specified as

follows:

R(0,;i- = x L(:,I-)d~/o U,;r)L. (5-1)

We impose several condition on the family p, the space Y, the space D of possible esti-

mates, and the loss function L.

Condition 5.1. The space Y is a Borel subset of a separable complete metric space and

FTy = B(y), where B(Y) is meant in the sense of the relative topology.

Recall that we assume there is given a probability space (R, Fo2, Q), where R = Y x X,

Fo~ = FTy x FTx, and Q2 is the joint distribution of Y and X. The marginal distribution of X,

denoted Qx, is an element of a family ~Px = (Pox : 8 E 8} of probability measures on (X, FTx),

where Pox(A) = Po(Y x A) for each Ae F x and 8 E 8. Similarly, the marginal distribution

of Y is an element of a family ~Py = {Po' : 0 E 8} of probability measures on (Y, FTy), where

Po ~ (A = Po(A x CL) I fo eac Aey andIC V t 8. Let denote a a-subalgebra of Fo.2 Let

f denote a nonnegative random variable. Let E( f |) denote the conditional expectation of f

with respect to the a-subalgebra W. Let Fa~x denote the a-subalgebra generated by the random

element X. Let E( f |X) = E( f |Fo~x). Let Ae F o.2 The conditional probability of the event

A given the random element X is denoted Pr(A|X) and is defined as E(I;|X), where Ig is the

indicator function of A. The conditional probability of the event A given that X = :r, which is

denoted Pr(A|X = :r), is any FTx-measurable function g for which

/ Ig()dQ)() g/~(:r)ilxdrx(r)foreah Ce (5-2)

To see that such a function indeed exists, the reader is referred to [34, pp. 220]. If y is a version

of Pr(A|X), then according to [34, pp. 221], a conditional probability of the event A given that

X = :r, g(:r), can be constructed as follows: g(X(w)) = g(w) (i.e., g(:r) = g(Lo), where w is an

element in R such that X (w) = :r).

Since in this work the distribution Q is unknown, but is known to be an element of P, the

conditional probability of Ae F o~ given X and the conditional probability of A given X = :r

depend on the true state of nature 8 E 8.

Definition 17. A faction Q (- |X) : FTy x R [0, 1], (A, w) H Q (A|X) (Lo) is said to be a

regular conditional distribution of Y given X if

1. For each we E Q(-|IX) (w) is a probability measure on (f Fy).

2. For each Ae F y, Q (A|X) is a version of the conditional probability Pr (Axx | X).

Fix 0 E 8. Condition 5.1 furnishes that there exists a regular conditional distribution of Y

give X wen Q= P [34, Theorem 2.7.5]. Let Po(-|X)l denote a regular conditionall disriutionvl

of Y given X when Q2 = Pa. For each r E and AeIC F y, we defineI Po(cA|:) = Po(A|X)(w),

where w E is such that X(w) = :r. We call Po(-|:r a regular conditional distribution of Y given

X = :r when Q = Po (or 8 is the true state of nature). Note that since Po(-|X) is regular, Po(-|:r

is a probability measure for each E X. In addition, for each Ae F y, Po(A|:r) is a version of

the conditional probability Pr (Axx | X = :r). Let pv x = { Po(- |:) : xrE X, 8 E 8 }.

Condition 5.2. There exist a o--Jinite measure p on (f FTy) and for each I E 8 a regular

conditional distribution of Y given X = :r when Q = Po, denoted Po (- |:r, such that Po (- |:r is

absolutely continuous with respect to ftfor each Ie 8 and xre X.

Let p~lo| I e) denote a density of Po (-|:r) with respect to p.

Condition 5.3. The loss faction Le E (FTx x B(D)) and is nonnegative.

LICondition 5.4. ThIe mpingLYIL 1 Po(A) E m(B(8)) for each Ae F y x FTx.

Conditions 5.3 and 5.4 were added to guarantee that the integration necessary in the

calculation of the risk function are well defined. It can be verified that if Conditions 5.1- 5.4

hold, R(-,;i) is an extended real-valued nonnegative B(8)-measurable function and hence the

Bayes risk relative to any distribution TrE Me/l is also well defined. We have the following

alternative expression for the risk function:

R(0, i) =~ LS (x.:)pol/ll)dP B] dox. (5-3)

Condition 5.5. The space D is a locally compact metrizable space and is o--compact.

We need the following condition for the case that D is not compact.

Condition 5.6. If D is not compact, then for each sequence of compact sets D, such that

U" ,D, = D and each element d, Sf D, (n = 1, 2, .. .), lim inf,,, L(x, d,) = supdED L(X, d)

for all x E X.

Condition 5.7. For each x e X, L(x, -) is lower semicontinuous on D.

It is easy to see that 9~ is subconvex. Indeed, given i:', i:" E 9~ and 0 < a~ < 1, let i:*

be an element in 9 that satisfies i*(-|y) = ak~'(-|y) + (1 a~)&"(-|y) for all ye E Then

R(-, i*) = a~R(-, i') + (1 a~)R(-, i"). Thus 9 is subconvex.

The space 9 can be identified with a subset of a certain vector space; this was shown

by LeCam in [30]. It is useful to discuss the properties of this vector space. The following

discussion essentially appears in [30] and [31]. Let C,(D) denote the class of continuous real-

valued functions on D with compact support, and let ||lu|| = supdED |U(d)|I for U E Oc(D).

Let LI1 denote the Banach space of equivalence classes of integrable functions on (Y, Ty, p)

with norm || f |1 = | f |dlp Denote by Lpv x the linear subspace of EL spanned by py lx. Let

the product space Co(D) x L~pv be equipped with the norm | | (u, f) | | = | |u| | V | | fl | for

ne E (D) and fe La x~. Let # denote the vector space of bounded linear functionals on

Oc(D) x L~pv The weak*-topology turns # into a locally convex topological vector space

[33]. Call gl, g2 EE Y|Tj) pX-eyUIValent if fr, |yl g2| fdy = 0 for all f e py x. Call

two estimators it,~ 2 ~ Y|X-equivalent if for each D' E B(D), Az(D'|-) and 2(D'|-)

are Py~lx-equivalent. L~et 9 denote the class of equivalence classes so obtained. A functional

e E is said to be positive if a > 0 and f > 0 imply ~(u, f) > 0. According to LeCam if

Conditions 5.1-5.5 hold, every positive linear functional of norm 1 can be represented by an

integral Q(u f ) = @~ u(d)di.~(dl~y) f (y~d y, where is E Certainly the converse is also true, i.e.,

each is E (or more precisely the equivalence class containing 2) is a positive linear functional

on C,(D) x LIv x of norm 1. Thus the class 9 is the class of positive linear functionals of

norm 1. In the sequel, we are not going to distinguish between an estimator and the equivalence

class containing this estimator. Whether a class A of estimators refers to the estimators or the

corresponding equivalence classes can be understood from the context. Let J denote the relative

topology for 9 induced by the weak*-topology.

Lemma 5.1. Suppose Conditions 5.1-5.5 and 5. 7 hold. Then the topological space (9, J) is

metrizable and R(0, -) is lower semicontinuous on 9 for all 8 E 8. If~ in addition, D is compact,

9 is compact.

Proof Under Condition 5.5, the space C,(D) is separable [30]. Since by Condition 5.1, Y

is a separable metric space, the space LI1 is separable [35, pp. 92]. Since LIv x is a subspace

of a separable normed space, it is also separable. It follows that the space C,(D) x L1pv x is

separable. By Theorem [36, Theorem 3.16], if @oc C is weak*-compact, then @o is metrizable.

By Banach-Alaoglu Theorem [33, Theorem 2.5.2], the set Be = {~ E # : || || < 1} is

weak*-compact, where | |~ | denotes the operator norm of a E Thus Be is metrizable. Since ]

is the relative topology, 9 is metrizable.

Let R(x, o, 2) = f L(x, 2)pa (1i| I )dpl. Then by (5-3), R(0, 2) = f R(x, 0, 2)dlox (x).

Using the results of LeCam [30], it can be shown that R(x, 8, -) is lower semicontinuous on 9

for each (0, x) E 8 x X. Since 9 is metrizable, we have by Fatou's Lemma that R(0, -) is lower

semicontinuous on 9 for each 8 E 8.

Suppose D is compact. LeCam [30] showed that a class A of estimators is J-compact if it

is J-closed and if the following conditions holds: For each e > 0 and each (0, x) E 8 x X, there

exists a nuE C:(D) satisfying 0 < Ir < 1 and fy u(d)dit(d y)dYo(,,I|, > 1 e unifo~rmly for all

i E A. The preceding condition certainly holds for 9 when D is compact. Since 9 is 1-closed,

it is compact. O

The following auxiliary lemma is needed in order to prove that Condition 4.2 holds under

very weak conditions.

Lemma 5.2. Let (A, F4~) be a measurable space. Let (C, w) be a topological space that is

locally compact, o--compact, and metrizable, but not compact. Let f : A x C R WU {+oo}

be a nonnegative function. Suppose f (a, -) is lower semicontinuous on C for each a s A.

Suppose for each sequence of compact sets C, such that U" ,C, = C and each element c, ( C,

(n = 1, 2, .. .), lim inf us, f (a, en) = supeec f (a, c) for all as A.~ Let (C*, 0*) denote the

one-point compactification ofC, and let oo denote the added point. Let f : A x C* R WU {+oo},

be defined as follows: For each as A f (a, c) = f (a, c) if ce C, and f (a, 00) = supeec f (a, c).

Then C* is compact and metrizable, f is nonnegative, f (a, -) is lower semicontinuous on C*

for each as A and a subset Co of C is in B(C) if and only if it is in B(C*). In addition, if

fe m(F x B(C)), fe m(Fa x B(C*)), and if f (, c) E m(A) for each Ee C,

f (-, c) EM (F~) for each c E C*.

Proof See Appendix A. O

Theorem 5.1. Suppose Conditions 5.1-5. 7 hold. Then Condition 4.1 and 4.2 hold for (8, 9, R).

As a consequence, there exists a minimax estimator and a restricted risk Bayes solution relative

to (-r, Co) for each -r Me Zl and Co > V(8, 9, R).

Proof We already showed that Condition 4.1 holds. Let us show that Condition 4.2 holds.

Suppose D is compact. Then by Lemma 5.1, Condition 4.2 holds. Suppose D is not compact.

We will use the one-point compactification of D to prove the theorem. The idea to use the

one-point compactification of the class of possible estimates to prove results of the type of this

theorem seems to appear first in [30]. Let D* denote the one-point compactification of D, and

let oo a D* denote the point that is added to D. Let L* : E x D* R WU {+oo} be defined

as follows: For each x E X, L*(x, d) = L(x, d) if d e D, and L*(x, 00) = supdeD L(x, d).

Let 9* denote the class of all estimators with D* as their space of possible estimates. By Lemma

5.2, L* E m(FTx x B(D*)) and D* is compact. Let L*(x, 2)(y) = JD* L*(x, d)di(dly),

and let R* (0, i) = fy xx L*(x, 2) (y)dYo(y, x). Then Conditions 5.1-5.5 and 5.7 hold f~or

(8, 9*, R*). Then by Lemma 5.1, / is compact and R*(0, -) is lower semicontinuous on 9*

for each 8 E 8. It can also be verified that R*(-, a) E m(B(8)) for each as E *. Let 9*(D)

denote the class of estimators & in 9* such that 2(Dly) = 1 for all ye E We claim that

(9*(D), R*) -4 (9*, R*). Indeed, fix i' E 9* such that i'(ooly) > 0 for some ye E Let

y' = {ye Y : i'(Dly) = 0}. Clearly Y' is measurable. Let (Aly) = i'(A n Dly)/i'(Dly)

for y ( y' (A E B(D)), and let 2(-|ly) be a Dirac measure with respect to a point d' E D

for all ye Y'. It can be verified that 2(D'|y) is B(y)-measurable for each D' E B(D).

Now if ye Y', then L* (x, 2) = L* (x, d') < L* (x, 00). If y ( y', then SD* L* (x, d) di(d|y)=

JD L*(x, d)/i'(Dly)di'(dly) < x'(Dly) JD L*(x, d)/i'(Dly)di'(dly)+ (1-i'(Dly))L*(x, 00) =

JD* L*(x, d)di'(dly). Thus L*(x, i) < L*(x, i'). It follows that & is as good as i'. This proves

that ( / "(D), R*) -4 ( / R*). Since 9* (D) C / ', we have that (9* (D), R*) ~ ( / R*).

Clearly ( /, R) ~ (9*(D), R*). Thus ( /. R) ~ ( /', R*). It follows that Condition 4.2

holds. O

As a consequence of Theorem 5.1, under the rather weak Conditions 5.1-5.7, there exists a

minimax estimator and a restricted risk Bayes solution relative to (-r, Qo) for each -r Me Zl and

Qo > V(8. /. R). In order to get the stronger results when 8 is compact, namely that (8, 9, R)

is strictly determined and that there exists a least favorable a priori distribution, we need to

prove that Condition 4.3 holds. Unfortunately, this seems to require rather strong conditions

on the loss function and family P. A set of such conditions is well known for the case that the

loss function is uniformly bounded and P is dominated by a o--finite measure. However, we

are mainly interested in the case that the loss function is unbounded (e.g., the quadratic loss

function). Moreover, in many cases P is not necessarily dominated by a o--finite measure.

CHAPTER 6

THE CASE OF CONVEX LOSS FUNCTION

In this chapter we consider the special case in which D is a convex subset of a finite

dimensional normed space and L(x, -) is convex over D for each x E X. Throughout this chapter

we will assume that L is a real-valued function, Y = RA~, X = WN, Ty = a(IN), and

-T = B(RWN) even if it is not implicitly stated. We are mainly interested in the case that the loss

function is quadratic, i.e., L (x, d) = | V(x d) | 2, where Ve R N, X Nz We will need the following

conditions.

Condition 6.1. for each x E X there exists an e > 0 and a c such that L(x, d) > eld| + c for all

dE D.

Condition 6.2. The measures {PY : 0 E 8 } are mutually absolutely continuous, i.e., for each

8, 8' E 8, PY is absolutely continuous with respect to Pf.

Clearly nonrandomized estimators are more attractive than randomized estimators in terms

of implementation. In general, randomized estimators can outperform nonrandomized estimators.

However, if D is a convex subset of R"N, and if for all x E X, L(x, -) is convex over D, it may

be sufficient to consider nonrandomized estimators. Let ~D denote the class of nonrandomized

estimators. The following lemma is closely related to the Rao-Blackwell theorem [37, Theorem

1.6.4].

Lemma 6.1. Suppose D is a convex subset of R", L(x, -) is convex over D for all x E X, and

Condition 5.3 holds. Then if Conditions 6.1 and 6.2 hold, ~D -4 9.

Proof Let is E Let Oo = {0 E 8 : R(0, 2) < +oo}. If 80 is empty, any estimator in ~D is

as good as 2. Suppose then that 0o is nonempty. Clearly the lemma is proved if we can show that

there exists an estimator i' E D such that R(0, i') < R(0, 2) for each 8 E 0o. Fix 0 E 80. Since

R(0, 2) < +oo, there exists a set Ae F y x &T such that L(x, 2)(y) < +oo for each (y, x) E A

and Po(A) = 1. Let C = {ye Y : (y, x) E A}. It follows that JD Id $dy) < +OO for 811 y 6 C.

Thus the integral JD d di(dly) is well defined for all y e C. It is well known that C E FTy. Clearly

Po(C x X) = 1. Thus Po'(C) = 1. Since the measures {Po' : 0 E 8} are mutually absolutely

continuous, Po'(C) =1 fort eac n 8~. Thu Po(C x y) 1 for each 8 E 8.

Let i' denote a nonrandomized estimator such that i'(y) = JD d di $ly) for y 6 C

and i'(y) = 0 otherwise. That i' is indeed a nonrandomized estimator, i.e., a FTy-measurable

function, follows from the fact that C E FTy By the Jensen inequality,

L~~x,2)(y)= L~~~idy ~~'y)) onC X (6-1)

Integrlating (6-1) with respectL to Po, we have R(0, 2) > R(0, i'). Since i' E ~D, ~D -4 9. O

Since ~D -4 9, we consider in the rest of this chapter the estimation problem (8, ~D, R)

instead of the estimation problem (8, '/, R). Clearly, ~D is subconvex by Jensen inequality.

Hence Condition 4. 1 holds for (8, ~D, R). Since ~DC 9, ~D ~ 9. Hence it is clear that Condition

4.2 holds for (8, D, R) if it holds for (8, 9, R). Thus if Conditions 5.1-5.7 hold, Condition

4.2 holds for (8, D, R). In Lemma 6.2 below, we show that under weak conditions, Condition

4.4 also holds for (8, ~D, R). In order to prove that Condition 4.3 holds for (8, ~D, R), it seems

necessary to make additional assumptions on the loss function and the family P.

Sometimes it is convenient to restrict the class of estimators that are available to the experi-

menter to the class of affine estimators. An estimator x is said to be affine if it is nonrandomized

and is an affine function on y. Since we consider only the case that Y = RWN and X = RAN, & is

affine exactly when & = Ay + b for some Ae R N X N and be R AN, and the space of possible es-

timates D is then RWN. Let L denote the class of affine estimators. The space L can be identified

with the space RWNXN x RWN where (A, b) E RWNX~y x RWN, is the estimator & = Ay + b E

and vice versa. Thus the space L can be identified with a finite-dimensional vector space with the

following addition and multiplication by a scalar: If & = (A, b), i' = (A', b') and a~ is a scalar,

& + i' = (A + A', b + b') and ai~ = (a~A, a~b). Let the space be equipped with the norm

|| "'| |A- '|| + b b'|i, where & = (A, b) and i' = (A', b'). Clearly is convex.

It is easy to see that Jensen's inequality furnishes that R(0, -) is convex over L for each 8 E 8

if L(x, -) is convex over D for each x E X. Thus if L(x, -) is convex over D for each x E X, L

is subconvex and hence Condition 4. 1 holds for (8, L, R). In Theorem 6.1 below, we show that

under rather weak conditions, Condition 4.2 holds for (8, L, R). As in the estimation problem

(8, ~D, R), it seems necessary to make further assumptions on the loss function and the family

p in order to prove that Condition 4.3 holds for (8, L, R). In Lemma 6.2 below, we show that

under weak conditions, Condition 4.4 also holds for (8, L, R).

Theorem 6.1. Suppose D = RWN, and Conditions 5.3, 5.4, 5.6, and 5.7 hold. Then Condition

4.2 holds for (8, L, R). As a consequence, there exists a minimax estimator and a restricted risk

Bayes solution relative to (Qo, r) for each Qo > V (8, 9, R) and -r E eZl.

Proof Clearly if {i,} E is a sequence that converges to an element i* in the sense of the

norm |I | | |, it converges pointwise on y. Let in = (A,, b,) for n = 1, 2, ... Suppose in

converges to i* = (A*, b*). Fix x E X. By Condition 5.7, lim inf,,, L(x, 2,(y)) > L(x, i* (y))

for each ye E By Fatou's lemma, lim inf,,, R(0, 2,) > R(0, i*) for each 8 E 8. Thus

R(0, -) is lower semicontinuous on for each 8 E 8. Let us show that for each sequence of

compact subsets C, of L such that U" zC, = L and each element in Sf C, (n = 1, 2, ...),

lim inf,,, R(8, in) = supy,~ R(0, i) for all 8 E 8. Fix a sequence of compact subsets C,

of L such that U" zC, = L and a sequence {i,} E L such that in ( C, (n =1,2...)

Fix 0 E 8. Certainly lim inf,,, R(8, in) < supper R(0, 2). Thus it is left to prove that

lim inf,,, R(0, in) > supy,~ R(0, 2). By Fatou's lemma,

lim inf R(0, 2,) > lim inf L(x, 2, (y))dPo(y, x). (6-2)

Fix (y, x) E Y x X. Let D, = {i,(y) : in E C,} (n = 1, 2, .. .). We claim that D, is a

compact subset of D. Indeed, let {di} be a sequence in D,. Then there exists a sequence {ij} in

C, such that (~(y) = di. Since C, is compact, there exists a subsequence {i~ } of the sequence

{ii} and an element i'* E C, such that &~ i '*. This implies that &~ (y) i '*(y), whence

{di, } i '*(y). Since i'*(y) E D,, D, is compact. We claim that U" zD, = D. Fix d E D.

Clearly there exists i s such that 2(y) = d (e.g., & = (A, b), where A = 0 and b = d).

Since U" ,C, = C, i s C, for a sufficiently large. This implies that d e D, for a sufficiently

large. Thus U" zD, = D. By Condition 5.6, lim inf,,, L(x, d,) = supdeD L(x, d), where

d, = 2,(y). It is easy to verify that supdeD L(x, d) = sup,,, L(x, 2(y)). Thus for each i s

we have

lim inf L(x, in(y)) > L(x, 2(y)). (6-3)

Since y and x are arbitrary,

Since x is arbitrary,

lim inf R(8, in) > sup R(0, 2). (6-5)

Since is a finite dimensional normed space, it is locally compact, o--compact, and

metrizable. Let denote the one-point compactification of L and let oo denote the added point.

For each 8 E 8 let R*(0, 2) = R(0, 2) if & E L, and let R*(0, 00) = sup,,, R(0, 2). By Lemma

5.2, is compact and metrizable, R*(0, -) is lower semicontinuous on for each 8 E 8, and

R*(-, a) E m(B(8)) for each as E *. Clearly L C *. Since oo is the only element in \ L and

R*(-, 00) > R(-, 2) for each is L (, R) -4 (*, R*). Since C *, (, R) ~ (*, R*). Thus

Condition 4.2 holds. O

The following lemma is concerned with the essential uniqueness of Bayes solutions.

Lemma 6.2. Consider the estimation problem (8, K, R), where X is either ~D or L. Suppose

D is a convex subset of R"N, L(x, -) is strictly convex over D for all x E ./, Condition 5.3 holds,

and V (8, K R) < +oo. Then if Condition 6.2 holds, a Bayes solution relative to -r Me Zl is an

essentially unique Bayes solution relative to -r.

Proof We prove the lemma for the case X = ~D. The proof for the case X = L is entirely

analogous. Fix -r Me Zl. It can be verified that our assumption that V(8, ~D, R) < +oo

implies that infeez, r (-, 2) < +oo. Suppose i', i" E D are Bayes solutions relative to -r.

The lemma is proved once we show that is equivalent to i'. Since inf~Er r(-r, i) < +oo,

r (-, i') = r (-, i") < +oo. Let i* = 1/22' + 1/22". Then by Jensen inequality, L (x, i* (y)) <

1/2L(x, i'(y)) + 1/2L(x, i"(y)) with strict inequality whenever i'(y) / i"(y). It follows that

R(0, 1i*) < 1/2R(0, 1i') + 1/2R(0, i"). Then clearly r(-r, i*) < 1/2r(r, i') + 1/2r(r, i"). Since

i' and i" are Bayes solutions relative to -r, we must have r (-, i*) = 1/2r (-, i') + 1/2r (-, i").

Thus R(0, i*) = 1/2R(0, i') + 1/2R(0, i") (a.e. -r). Clearly there exists a set 0o E B(8) such

that R(0, i') < +oo and R(0, i") < +oo for each 8 E 80 and -r(80) = 1. Thus there exists

an element 8o E Oo such that R(00, i*) = 1/2R(00, i') + 1/2R(00, i"). It follows that there

exists a set Ae F y x FTx such that L(x, i*(y)) = 1/2L(x, i'(y)) + 1/2L(x, i"(y)) for each

(y, x)It EAIC an Poo(,A) =. Let C = {ye Y : (y, x) E A}. Then Ce F y, ~I = ~I" on C and

Poo(C x X) = 1. Since the measures {Po' : 0 E 8, x E X} are mutually absolutely continuous,

Po(C x X) = 1 for each 8 E 8. It follows easily that i" is equivalent to i'. O

In the rest of this chapter, we assume the loss function is quadratic, i.e., L : (x, d) H

|V(x d)|12, where Ve R N"X A. We assume VTV > 0. The extension to the case VTV > 0 is

discussed later. In this case it can be verified that Conditions 5.1, 5.3, 5.5-5.7, and 6.1 hold. Thus

if D is convex and Condition 6.2 holds, ~D -4 9. Certainly ~D is subconvex and Condition 4.1

holds. In addition, if Conditions 5.2 and 5.4 hold, Conditions 4.2 holds for (8, ~D, R). Similarly,

Condition 4.1 holds for (8, L, R), and if Conditions 5.2 and 5.4 hold, Condition 4.2 holds for

(8, L, R). Suppose, in addition, that P is a Gaussian family of distributions, i.e., Y and X are

jointly Gaussian, when 8 is the true state of nature, for each 8 E 8. Then py and pv x are also

Gaussian families of distributions. Suppose the family pylx is dominated by the Lebesgue-

Borel measure on (RW~y x B(RIWy)), which is denoted p. It can be easily verified that py is

also dominatedLL by p andC thatL sinlce, in1 addUition, for echLI t V po~y), the densitLy of1, Po:,+, 1~,., 1,,,,:,~+, ,,,,Dr/\,/. 1,~,:,, kwith

respect to p, is positive, the measures {Po' : 0 E 8} are mutually absolutely continuous. Thus

Condition 6.2 holds. We would like to check under what conditions Condition 4.3 holds. Under

the current assumptions, it is well known that there exists a regular conditional distribution of X

given Y =. y,~, when is the true state of ntreLt Po(-|y) denote thi conditional distributor,:

which is well known to be Gaus sian. Let is : Ya R Nz y H Ea(X |y), where Ea (X | y

denotes the conditional expectation of X given Y = y, when 8 is the true state of nature. That

is, is is the conditional mean estimator for x based on the observation y when Q = Pa. Let

Fe = Ea(|V(fo(Y) X)|2), i.e., ~e iS the mean squared error (MSE) matrix of is, when

Q2 = Pa. Then by the so-called orthogonality condition, it is straightforward to get the following

expression for the risk function:

R(0, 2) = tr(V~oV )+|To) 2(y|pe(y)dy.l (6-6)

In what follows 9, is the class of Bayes solutions when (8, ~D, R) is considered, i.e., for each

i* AD tereexitsren uhta i,7 inf~Ez r (i, -r). Since ~D -4 9, each element

in 9, is also a Bayes solutions when (8, 9, R) is considered. In what follows, continuity of

functions from 8 into R"N and RWNxN is meant in the sense of the Euclidean norm and 2-norm,

respectively.

Theorem 6.2. Let Z = [YT XT]T. Suppo3se H Ea(Z) and 8 H Ea(ZZT) are continuous

on 8. Then if 8 is compact, R(-, 2) is lower semicontinuous on 8 for each f e D and R(-, 2) is

bounded and continuous on 8 for each is E Hence Condition 4.3 holds for (8, ~D, R).

Proof As mentioned earlier, py is a Gaussian family. It is easy to see that by the assumption

of the theorem, 8 H Ea(Y) and 8 H Ea(YYT) are continuous on 8. Since we assume

that the family py is dominated by p, 8 po(y) is continuous on 8 for each ye E It is

well known that 20 (Y) = Ea (X) + Ea ((X Ea (X)) (Y Ea (Y))") Ae (y Ea (Y)) and

re = ro Ea((X Ea(X))(Y Ea(Y))")Ae E((Y Ea(Y))(X Ea(X))T), where As

and To are the covariance matrices of Y and X, respectively, when 8 is the true state of nature

(i.e., when Q = Po). Since 8 H Ea(Z) and 8 H Ea(ZZT) are continuous on 8, 8 H fo(y) is

continuous on 8 for each ye~ E and 8 H Fo is continuous on 8. Since Po is Gaussian, there

exist a matrix As a RWN, X and a vector be E RWN such that 20 (Y) = Aey + be. Since 8 H 20 (y)

is continuous on 8 for each ye~ Y and Y = RIWN, 8 H As and 8 H be are continuous on 8. Since

8 po(y) is continuous on 8 for each ye E it follows easily from Fatou's lemma and (6-6)

that R(-, ) is lower semicontinuous on 8 for each is ED.

Fix -r E Mo.l Let 2, denote the (essentially unique) Bayes solution relative to -r. It is well

known that when the loss function is quadratic, the conditional mean estimator with respect to

-r Me Zl is a Bayes solution relative to -r. Thus without loss of generality, we may take 2, to

be the conditional mean estimator, i.e., 2,(y) = E,(Xly), where E, denotes the expectation

operator when 8 is the value taken by a random element whose distribution is -r. Note that since

by our assumption V(8, ~D, R) < +oo, r(-r, 7) < +oo. This implies that the MSE matrix of the

estimator 7 is bounded.

Clearly E, (X |y) = E, (E, (X |y, 8) | y). Let us examine the term E, (X |y, 8). This term is the

conditional expectation of X given Y = y and 8, where 8 is the value taken by a random element

whose distribution is -r. But this is exactly Eo (X |y). Thus E, (X |y) = E, (Eo (X |y) |y). It follows

that | V(7(y) 200 (Y)) 2" = | VE, (So(Y) 200 (Y) Y) 12. Thus

I ~v(v)(Y f o.(Y) 2 < E,(|V(f o(y) iso lU)) 121

=E,(|V((Ae Aeo)y + be boo) 21v

< E,((||V||2(|| Ao Aeo 2|lay| + |bo boo 1))21Y

Since the mappings 8 H As and 8 H be are continuous on the compact set 8, they are bounded.

Thus there exist positive real numbers cl and c2 Such that

|V(:, (y) ieo(Y) 12 < E,((clly| + c2 2 y) = (Cly + C2 2

Since 8 H Fo is continuous on 8, which is compact, there exists a positive real number a such

that tr {V~oVT} < a for all 0 E 8. We have from (6-6) that

R(00, 7)

It is easy to verify that R(00, fr) < +oo. Put h(y) = (clly| + C2 2. Since 8o is arbitrary

R(0, 7) < +oo for each 8 E 8. Moreover, for each 8 E 8, |V(i,(y) is(y))|2 < h(y) and & is

a Po-integrable function. Certainly

|R(0, 17) R(00, 97)| < | tr(V(Fe Foo)VT)|I

+/4 (W(iaiii) -2,(7)| -|IooU -7U) 2 0

Let {0,} be sequence that converges to 8o. We showed that lims,, |Co. Feo | = 0. Thus

lim tr(V(Fos Foo)VT) = 0. (6-9)

By the Lebesgue dominated convergence theorem,

Certainly for each n > 0, | V(ion (y) 97(y)) 12 < h(y). Since h is Poo-integrable and 8 po(Y)

is continuous on 8 for each ye E a well known theorem on exponential families [37, Theorem

1.4. 1] furnishes that

It can be verified that since 8 po(y) is continuous on 8 for each ye E the above equation

implies that

Since |V(fos (y) fr(y))|2 y)

/ | ~ion(7)- 977))2 00 y)|d 0.(6-11)

By (6-8)-(6-11), R(-, 7) is continuous on 8. Since R(-,, ) < +oo and R(-, 7) is continuous

on 8, which is a compact set, R(-, 7) is bounded on 8. It follows that R(-, 2) is bounded and

continuous on 8 for each is 9 O

Let Mr denote the class of Bayes solutions when (8, L, R) is considered.

Theorem 6.3. Let Z = [YT XT]T. Suppo3se H Ea(Z) and 8 H Ea(ZZT) are continuous

on 8. Then if 8 is compact, R(-, 2) is lower semicontinuous on 8 for each is L R(-, i) is

bounded and continuous on 8 for each is M Hence Condition 4.3 holds for (8, L, R).

Proof Although some modifications are needed, the proof of this theorem is very similar to the

proof of Theorem 6.2 and is omitted. O

While the proofs of some of the results of this chapter clearly break down if VTV is not

positive definite, all these results are valid also in the case that VTV is not positive definite.

There is a simple method to show that this is indeed the case. Note that if VTV is not positive

definite, L(x, d) = L(x, d') whenever d d' E Ni~(V). Thus we may call d and d' in RWN

equivalent if d d' E Ni(V) and choose the space of possible estimates to be the set of

equivalence classes so obtained instead of RWN. In this case, the space of possible estimates is

equipped with the metric a (d, d') = | Vt V(d d') |, where d, c' E D, d is any element in d, and

d' is any element in d'. This choice for D is equivalent to choosing D = Ni(V)I with the usual

Euclidean norm since for any equivalence class in D there is associated a point in Ni(V)I and

vice versa. It can verified that with either one of these choices for D, the results for (8, ~D, R) are

still valid. To show that the results for (8, L, R) are still valid, it is necessary, to define a class

cL = {(N~(V)IA, Ni(V)lb) : (A, b) E }. It is obvious that L' ~ L. Certainly for each & E L',

2(y) is in the new space of possible estimates. Now, it is straightforward to show that all the

results of this chapter are still valid for (8, L', R) and hence for (8, L, R).

CHAPTER 7

FINDING A MINIMAX ESTIMATOR AND THE DUAL PROBLEM

In this section, we consider in more detail the problem of finding a minimax estimator for an

estimation problem (8, X, R). We need the following additional conditions.

Condition 7.1. For any -r M E Zl, there exists an essentially unique Bayes solution relative to -r.

We let 2, denote the (essentially unique) Bayes solution relative to -r. Let r(-r) = r(-r, 7).

Let Me/l denote the class of distributions in Me/l with finite support.

Condition 7.2. If {n } is a sequence in Me/l that converges weakly to -r Me Zl, then R(0, in)

converges to R(0, 7) uniformly on compact subsets of 8.

By Corollary 9.1i, if Conditions 4.1-4.4 hold and 8 is compact, the problem of finding a

least favorable a priori distribution is dual to the problem of finding a minimax estimator. Thus in

the rest of this chapter, we concern the problem of finding a least favorable a priori distribution.

The following theorem is essentially similar to a theorem in [18] and the iterative algorithm

proposed in [38].

Theorem 7.1. Suppose 8 is compact, R(-, i) is continuous on 8 for each is 9 and Conditions

7.1 and 7.2 hold. Construct a sequence {-ri}@, E Me/l as follows. Let -ri be any distribution in

Me/l. Having chosen -rl, ri E M, let Os E 8 be such that R(0s, in) = supeoe R(8, in). Let

ni,o = c004 + (1 a~)nr. Let asi be such that r(9i,ai) = supe[o, 1] T(ni,a) and let ni+l = 74,ai. Then

the sequence {8 }) converges weakly to a least favorable a priori distribution.

Proof The proof follows easily from the proof of [18, Theorem 2.3], with slight modifications.

The main difficulty in the algorithm described in Theorem 7.1 is in finding 04 E 8 such that

R(0s, in) = supoe, R(8, in) for i > 1. Another difficulty is due to the fact that, in general,

since the support of -r may grow as i grows, the complexity of the algorithm calculations also

grows with i. The problem of finding asi such that r"(nr,ai) = supe[o,1] "(ri~,a) is addressed

below and can be solved numerically. In some special cases, it is easy to find Os E 8 such that

R(0s, in) = supose R(8, in) for i > 1 and the complexity of the algorithm calculations remain

fixed as i grows. In these special cases, the algorithm described in Theorem 7.1 is practical in

finding minimax estimators. One such case is when 8 is a finite set. In the sequel, we show that

when we consider linear estimation with quadratic loss function, the algorithm of Theorem 7.1

can be often used to find a minimax estimator. In the more general case, this algorithm can be

used just to find e-minimax estimators, which are good approximations to minimax estimators for

e sufficiently small. We discuss the derivation of e-minimax estimators, in great detail, in the next

chapter.

The problem of finding asi E [0, 1] such that r"(40,a) = supae[o, 1] T(8i,a) is a standard

optimization problem in RW:

maximize r"(cl + (1 a~)-r) subject to as [0, 1].

Fix -r1,r BE Me/l, and let To, = 071r + (1 a~)-r for as [0, 1]. In the rest of this chapter we

consider the following optimization problem, which includes the previous one as a special case:

maximize r(-r,) subject to as [0, 1]. (7-1)

Lemma 7.1. a~ H (-r) is concave on [0, 1].

Proof That r is concave on Me/l is well known and easy to prove. As a consequence, it is easy to

verify that a~ H (-r) is concave on [0, 1]. O

Let D(a~) = r(Tis iv.) r(-r,;i'r) for as [0, 1].

Lemma 7.2. Fix c~o E (0, 1). Then r"(-r) r"(To.) < D(no)(a~ n~o) for all as [0, 1].

Proof Certainly r(-r,) r(To,,) < r(-roo 'r,) r(-r,,). It can be easily verified that r(-roo m>)

r(-r,,) = (a~ n~o)D(n~o). The lemma follows easily. O

As a consequence of this lemma, -D(n~o) is the subderivative of a~ -r(-r,) at the point

n~o E (0, 1).

Lemma 7.3. Suppose Condition 7.2 holds. Then a~ D (c) is monotonically decreasing and

continuous on [0, 1] and r(-r,,) = supe[o,11? (-r) for n~o E [0, 1] if D(n~o) = 0.

Proof Certainly To, converges weakly to -roo whenever a~ converges to cto. Since -r1 and -r2 have

finite supports, Condition 7.2 furnishes the continuity of a~ D(a~) on [0, 1]. Since -D(a~) is

the subderivative of the convex function am -r(-r,), aa D(a~) is monotonically decreasing

on (0, 1). By continuity, a~ D(a~) is monotonically decreasing on [0, 1]. Fix to E [0, 1].

Suppose D(ao) = 0. Then by lemma 7.2, r(-r,) < r(-roo) for all as [0, 1]. It follows that

r(To,) = supago,1,] r(To,).

Remark 7.1. Lemmas 7.2 and 7.3 imply that we can use relatively simple numerical al gwiduallr\

to solve (7-1) numerically. Suppose r (-ri, 27) = r (-72, 7). Then r (-r1) = r (-ro, 7) for all

as [ 0, 1]. It follows that r (-r1) > F (-r) for all as [0, 1] and hence a~ = 1 is a solution of (7-1).

Similarly, if r (-r, in ) = r (-r2, i), a~ = 0 is a solution of (7-1). Using the fact that a~ D (a)

is continuous and monotonically decreasing on [0, 1], and the fact that as [0, 1] is a solution of

(7-1) if D(a~) = 0, the optimization problem (7-1) can be solved as follows: If D(1) > 0, then

a~ = 1 is a solution of (7-1). IfD (0) < 0, then a~ = 0 is a solution of (7-1). Finally, ifD (0) > 0

and D (1) < 0, there exists as (0, 1) such that D (a) = 0 and hence a~ solves (7-1). In ;Ihi\ case,

since D(a~) = 0 and aa D(a~) is monotonically decreasing on (0, 1), a~ can be easily found

using the bisection method.

CHAPTER 8

APPROXIMATING A MINIMAX ESTIMATOR

In this chapter we approximate minimax estimators by using e-minimax estimators. The

main idea is to find a minimax estimator (or an approximation to a minimax estimator) i* for an

estimation problem (87, X, R), where 87 is a finite subset of 8, such that & is an e-minimax

estimator for the estimation problem (8, X, R). The main question is how to construct the set

87. Finding a minimax estimator for (87, .F. R), when Of is a finite set can be done using the

algorithm in Theorem 7.1. In practice, it may be necessary to find an e'-minimax estimator for

(87, K, R) such that this estimator is an e-minimax estimator for (8, X, R). In the latter case,

the algorithm in Theorem 7.1 can still be used, but it is necessary to have a condition that enables

us to check when the required precision is achieved. Checking if an estimator 7, an essentially

unique Bayes solution relative to -r, is an e-minimax estimator can be done using the following

lemma:

Lemma 8.1. Suppose Condition 7.1 holds and supose R(8, 17) F (-) < e, then 2, is an

e-mmnimax estimator

Proof Since supose R(8, ir) in a supose R(0, i) > r(-r), we have that

sup R(0, 7) < inf sup R(0, 2) + e.

Thus 2, is an e-minimax estimator. O

The condition of Lemma 8.1 is only a sufficient condition for an e-minimax estimator.

However, under certain conditions, if the numerical algorithm converges to a least favorable a

priori distribution, then there exists an integer NV such that Lemma 8. 1 is satisfied for the NVth

iteration.

Lemma 8.2. Suppose 8 is compact, ./ is subconvex and has the property (W*), R(-, i) < +oo

for each is E R(-, 2) is continuous on 8 for each is E and Conditions 7.1 and 7.2 hold.

Let {nri } EZ/e be a sequence that converges weakly to a least favorable a priori distribution

I-o EM eZl. Then there exists an integer NV such that supose R(8, i,) r (-r) < e for all i > NV.

Proof Clearly the lemma is proved if it is shown that supose R(8, in) r"(-r) 0 Since 8 is

compact and Condition 7.2 holds, it is straightforward that

sup R(0, in) sup R(0, fro). (8-1)

BEe sEe

Since R(-, fr) is continuous on 8, it can be shown, as in the proof of [38, Theorem 2 part 3],

that

F(i) r(To). (8-2)

By Theorems 4. 1 and 4.5, there exists a minimax estimator and (8, K, R) is strictly determined.

By Theorem 4.6, fro is a minimax estimator. Thus

r(-ro) = sup R(0, fro). (8-3)

BEe

By (8-1)- (8-3), supose R(8, in) r"(-r) 0 and the lemma is proved. O

The following condition is needed for some of the results of this chapter.

Condition 8.1. 8 is a subset of a normed space 7 and the metric a for 8 is induced by the

norm |I | | | of W.

A set U is said to be 6-dense in 8 (in the sense of the metric a) if for any 8 E 8 there exists

8' E U such that A(0, 8') < 6. Note that if 8 is compact, for each 6 > 0, there exists a finite

subset of 8 that is 5-dense in 8.

Theorem 8.1. Suppose X is subconvex and has the property (W), Condition 7.1 holds, and the

family {R(-, 7) : -r Me }Zl is equicontinuous on 8. Then for any e > 0, there exists a 6 > 0

such that for any finite 6-dense subset 8; of 8, the following hold:

1. For any -r M E Zl, there exists a probability measure -ro E Te, such that

R(0, fro) R(0, 7) < e for all 8 E 8.

2. There ex-ists a probabiiliy measure 7o E 7e, such that 7 (7o) = supEreve 7 (7) and f r is an

e/2-minimax estimator

Proof 1) Let p(0, 8') = supee |R(0, 7) R(0', 7)|i. The equicontinuity of {R(-, 7):

-r Me}i/o on 8 implies that for any e > 0, there exists a 6 > 0 such that p(0, 0') < e/2

whenever A(0, 8') < 6. Let Of be a finite subset of 8 that is 5-dense in 8 in the sense of a.

Then 87 is e/2-dense in 8 in the sense of p, i.e., for any 8 E 8, there exists a O' E 87 such that

p(0, 8') < e/2. Fix -r Me Zl. Consider the estimation problem (87, X, R). Let 87 be equipped

with the discrete topology. Since 87 is finite, it is compact and R(-, 2) is continuous on 87 for

each is K It can be shown, similarly to Theorem 4.7, that there exists a probability measure

I-o E Te, such that R(0, fro) < R(0, 7) for all 8 E 87. Since 87 is e/2-dense in 8 in the sense

of p, R(0, fro) < R(0, 7) + e for all 0 E 8 and part 1) is proved.

2) Clearly the results of Chapter 4 can be used for the estimation problem (87, X, R). Thus

for any finite 87, there exists a least favorable a priori distribution Tro E -re, and fro is an admis-

sible minimax estimator for (87, X, R). It follows that maxeee, R(8, fro) < maxeee, R(8, fr)

for any -r Me Zl. Since Of is e/2-dense in 8 in the sense of p, supose R(8, iro)

supeoe R(0, 7) + e/2. Thus fro is an e/2-minimax estimator for (8, X, R). O

Remark 8.1. Theorem 8. 1 part 1) is an e-complete class theorem, provided that 9J is essentially

complete (the reader is referred to[/39]for the definition of an e-complete class of decision

functions). It differs from TU 10 al~ il: 's e-complete class theorem in[/39] and has the advantage

that each element in thri\ class is admissible.

Theorem 8. 1 part 2) implies that an admissible e-minimax estimator can be found in the

following way: Partition the set 8 to disjoint sets 81,82.., O N Such that the diameter of

84 (i = 1,. ., NV) does not exceed 6. In each set 84 take a point Bi. Let 87 = {Or 82, HN *

Solve for a minimax estimator for (87 X R). If 6 is sufficiently small, then the resulting

estimator is e-minimax for (8, K, R). Theorem 8.1 does not spec~if\ how to choose 6 that would

guarantee a certain e. However it is clear from the proof that it is sufficient that 6 \aiisfit 1

|R(0, 7) R(0', 7) | < e/2 whenever -r Me Zl, 8, 8' E 8, and A(0, 8') < 6. In fact for

thri\ choice of 6, it is enough derive an e/2-minimax estimator for (87 K R) and the resulted

estimator is e-minimax for (8, X, R).

The hypothesis in Theorem 8. 1 that the family {R(-, 7) : -r Me}i/o is equicontinuous on

8 is rather strong. While it is often satisfied in the case that the risk is based on a loss function

that is uniformly bounded, it may not be satisfied in the case that the loss function is unbounded.

Another, difficulty with Theorem 8.1 is that the requirement from the set 87 is very strong. A

set 87 that is constructed according to this theorem has the property that any estimator can be

approximated by a Bayes solution relative to a measure with support in 87 with degradation of

no more than e. However, we are mainly interested in approximating a minimax estimator and

not any estimator. Thus it may be possible to choose a finite set whose cardinality is significantly

smaller than 87 in Theorem 8.1. Due to the above, we are not going to use Theorem 8.1 in the

sequel, and we are going to derive methods that do not require equicontinuity.

It is convenient to use the notion of Fr~chet differentiability. Let B(X, Y) denote the set

of bounded linear operators from a normed linear space X to a normed linear space Y. Given

f E B(X, Y), || f || denotes the operator norm of f i.e., || f || = sup,: ||2llst ||I fX|| .

Definition 18. Let X and Y be normed linear spaces, U C X open, f : X Y and x e U. The

function f is said to be Fri'chet differentiable at x if there is an element AE B (X Y) such that

||f (x h) f (x) Ah||

him = 0.

hwo ||h||

We call A the Fri'chet derivative of f at x and denote it by D f(x).

If f : X Y is twice Fr~chet differentiable at x E X, we let D2 f(x) denote the second

Frichet derivative of f at x. Note that D2 f(x) E B(X, B(X, Y)). Given is 9 X, let DRa(0) and

D2Ra(0) denote the Fr~chet derivative and the second Fr~chet derivative, respectively, of R(-, 2)

at 0.

Definition 19. Given an estimation problem (8, X9, R), let y > 0 and 80 be a subset of

8. An estimator i* E X is said to be a (y, 80)-optimal estimator if supesea R8, *

(1 + y) V (o, K R). An estimator i* e X is said to be a y-optimal estimator if it is a

(y, 8) -optimal estimator

Clearly a y-optimal estimator is an e-minimax estimator for e = y V(8, K, R). It is often

more convenient to search for a y-optimal estimator instead of an e-minimax estimator. The

reason is that by deriving y-optimal estimator, 100y gives us the maximum degradation in terms

of the maximum risk in percent (relative to the minimax risk). This is different than the case that

we derive an e-minimax estimator, in which e is not normalized by the minimax risk. Certainly

what is considered a small e in a certain problem, may be considered huge in a different problem.

Lemma 8.3. Suppose X is subconvex and has the property (W), Conditions 7.1 and 8.1 hold

and 8 is compact. Suppose for any -r Me Rl, there exists an extension of R(0, 7) from 8 to an

open convex set 8 C UC c such that R(0, 7) is Fri'chet differentiable on U and there exists

a real number M~ such that | |DR,7 (0) | | < M~ supose R8, 17) for all -rE Me/l and 8 E 8.

F~ix Y > 0 and 0 < ?' < -i, and let d = .~i~ Then if 8p is a fnite 5-dense subset of 8, a

(y', 87) -optimal estimator is a y-optimal estimator

Proof Let -ro E Me/l be such that the support of -ro is contained in 87 and xfo is a (y', 87)-

optimal estimator. Fix 8, 8' E 8 such that | |8 el'|~ |, <. Then by the mean value theorem,

|R(0, ro) R(0', r)I I 0-0 8' supe[o~, ||DR;~b(a0B+ (1 a)0')| < 6MsuposeR(8 N~,r).

TIhus supose R(B, ir)(1 bM) < supose, R(B1 fro) < V(87, K^, R)(1 + y'). Since

V(87, K, R) < V(8, K, R), we have supose R(8, fro) < V(8, K, R)(1 + y), and fro is a

y-optimal estimator. O

The cardinality of a finite subset 87 of 8 that is constructed according to Lemma 8.3 may

still be very large; this may cause the calculation of a least favorable a priori distribution for

(87, X, R) to be formidable. In a special but very important case of compact 8, it is possible to

derive a finite 6-dense subset of 8 such that 6 depends linearly on j This means that if y is

decreased by factor of say 4, 6 should be roughly decreased only by factor of 2 (assuming y

and y'

Definition 20. Let K( be a subset ofa vector space V. A nonempty set S C K( is called an

extreme set of K if a, b E K, O < t < 1, and (1 t) a + tb E S imply a, b e S.

Definition 21. Let K be a subset of a vector space V. The point p e K is an extreme point of K

if and only if a, b e K, O < t < 1, and (1 t) a + tb = p imply a = b = p.

Let A be a subset of a vector space V. Let dA denote the extremal boundary of A, which is

the set of all extreme points of A. Let co(A) denote the convex hull of A. Given a subset K of a

vector space V and a subset S of K, let & (S) be the set of all points s ES such that a, b E K,

O < t < 1, and a = (1 t)a + tb imply a, b E S. Clearly the set &K(S) is an extreme set of

Kt. Given a set A, we let 2A denote the power set of A, i.e., the collection of all subsets of A. We

consider the case that 8 is a compact convex subset of the normed space C7 and 88 is a finite set.

Note that by [36, Theorem 3.20], the set co(88) is compact and hence the Krein-Milman theorem

[36, Theorem 3.23] implies that 8 = co(88).

Lemma 8.4. Suppose 8 is a compact convex subset of the normed space 7 and 88, is a finite

set. Let 6 > 0. Then there exists a finite subset 8; of 8 such that for any nonempty A E 2ae,

87 n deo (co(A)) is a 6-dense subset of deo (co(A)). The set 87 is a 6-dense subset of 8 and

Proof Let Al, A2,..., AN denote the elements of 2ae excluding the empty set. Fix 1 < i < NV.

It is easy to see that Beo(co(Ai)) C 8. Thus Beo(co(Ai)) is relatively compact and hence totally

bounded. Thus there exists a finite subset 87,< of deo(co(Ai)) that is 5-dense in de(co(Ai)).

Let 87 = U~,Of87. Then clearly 87 is finite. Moreover, 87 n de(co(Ai)) = 87,4 and hence

Of n Be(co(Ai)) is a 6-dense subset of Be(co(Ai)). Since 8 = co(88), 8 = de(co(88))

and Of is a 6-dense subset of 8. Let 8 E 88. Then {0} E 2ae. Since 8 is an extreme point,

Be(co({0})) = {0}. Thus there exists 1 < j < NV such that 87,; is a nonempty subset of {0} and

we must have B y,; = {0}. Thus 8 E Of and Of > 88. O

Let ?" = {ae R : ||a||1 = 1}.

Lemma 8.5. Suppose 8 is a compact convex subset of the normed space 7 and 88 is a finite

set. Let Or 82 N denote the elements of 88,. Fix 0 E 8. Then there exists an index set

J C (1, 2,. Nsuch that

#(i) > 0, i EJ

pe R" n ",O (i04- (8-4)

i=1 P(i) = 0, otherwise

Proof Certainly 8 can be written as a convex combination of 81, 82, *, HN. Let 1 < M~ < N be

ther largest intege, r for whichl therem exist ye R r n 7" and an index set J such that | J| = M~,

O 2", O()s ()>0frali ,ad#i for all i ( j. Suppose (8-4) is false.

Then there~ exists R' r R"P n 7"N and a non empty index set J' / J such that 8 = 'i)s

P'(i) > 0 for all is J ', and P'(i) = 0 for all i ( J'. Let po = (4+ #'/.Thn8= N )

Po(i) > 0 for all i EJ U J', and Po(i) = 0 for all i ( J U J'. Since J' / J, | J U J'| > M~, which

is a contradiction to the definition of M~. Hence (8-4) must hold. O

Lemma 8.6. Suppose X is subconvex and has the property (W), Conditions 7.1 and 8.1 hold,

8 is a compact convex subset of the normed space 7, and 88~ is a finite set. Suppose for each

-r Me Zl, there exists an extension of R(-, 7,) from 8 to an open convex set 8 C U cC

such that R(-, 7) is twice Fri'chet difgerentiable on U and there exists a positive real number

M~ such that | |D2R~, (0) | | < M supose R(8, ir) for all -rE Me/l and 8 E 8. Let y > 0 and

0 < y' < 7. Let a = .I Suppose 87 is a finite set that latirisi 1: For any nonemnpty

A E 2ae, Of n Be,(co(A)) is 5-dense subset of de(- (co(A)). Then a (y', 87)-optimal estimator is a

y-optimal estimator

Proof Let Or, 82, N denote the elements of 88~. Let -ro E Me/l be such that the support of

-ro is contained in 87 and xro is a (y', 87)-optimal estimator. Clearly R(-, fr) is a continuous

function on a compact set 8. Thus there exists a point 0* E 8 such that supos A, iRo8~)

R(0*, fr). If 0* E 87, then the lemma clearly holds. Suppose, then, that 0* is not in 87.

By Lemma 8.5, there exists an index set J C {1, 2, .. ., N}) such that (8-4) holds for 0*. Let

A = {Os : ie J}~. Since 0* is not in 87, 0* is not an extreme point (Lemma 8.4) and | J| > 1. Let

8' E Of n Be(co(A)) be such that A(0*, 0') < 6. Then 8' = Ci, y(i)8i for some y E R"~ n 7"N

such that y(i) = 0 for i ( J. Since 8* = C (i8fosmeeR N sctht(i>0

for all i EJ and P(i) = 0 for i ( J, 0* + b(0* 8') E co(A) for sufficiently small b > 0. Let

0"1 = 0* + b(0* 8'), where b > 0 is sufficiently small so that 0"1 E co(A). Then the line segment

connecting 8' and 0"1 lies in co(A). Moreover, 0* lies in this line segment. Since 8* is a maximum

of R(-~, o) over 8 and R(-,, fr) is continuously differentiable on an open set 8 C UC C7, the

directional derivative of R(-, fr) at the point 0*, in the direction of 8', must be zero, i.e.,

DR;,, (0*) (0'- 0*) = 0.

By the Taylor theorem,

|R0'ir) R0*ir l 2|' 0*||I sup ||D2 ;~~c81+ t)8*)1

2~aE[o, 1]

By the hypothesis of the theorem,

| | 8' 0* | |~ sup | | D2R ~(c81 1 c) 8* I 2 M sup R (0, iro)

aE[o, 1] aEe

Thus supose R(H, iro,) supose, R(H 70~) < 70,~ ) R('o~~) < 2M/2 supose R(H, iro)-

It canl be verified that since supose, R(H 0) < V(87, K,~- R)(1 + y') and V(87, K,: R) <

V(8, .[, R), then supose R(8, fro) < V(8, X. R)(1 + y). Thus fro is a y-optimal estimator.

Suppose the normed space C7 in Condition 8.1 is R"N with its usual norm. In this case, if 8 is

a compact convex set and 88 is finite, 8 is said to be a convex polytope in R".

Definition 22. Let Ve R NxN denote an odopels~rnal matrix and let I denote the ith column

vector of V. Let 6 E R"N be a vector whose ith component 6(i) > 0 for i = 1,. ., NV. We say

that A' is (6, V) -dense subset ofAC R "W if for any as A there exists an a' E ~A' such that

| (a -a')TI | < 6(i)for i= 1,...,N1.

We have the following version of Lemma 8.6.

Lemma 8.7. Suppose .F is subconvex and has the property (W), Conditions 7.1 and 8.1

hold, and 8 is a convex polytope in R"N. Suppose for each -r E f.... there exists an extension

of R(-, 7) from 8 to an open convex set 8 C U cC such that R(-, 7) is twice Fri'chet

difgerentiable on U and there exists a positive definite Ao a RWNxN such that -D2R~ (8

Ao supose R(0, 7) for all -r Me o and 8 E 8. Let V be a matrix whose ith column is the ith

eigenvector of the matrix Ao (i = 1,. ., NV). Let (4 denote the ith eigenvalue of the matrix Ao

(i = 1,. .l. N. Le 7 > 0 annd 0 < 7' < 7. Letd t IWZ hR be sh that 6(i) = \i

for i = 1, ... N. Suppose that 87 is a finite set that \nat~is7 \.: For any nonempty A E 2ae,

8; n de (co(A)) is (6, V)-dense subset of de -(co(A)). Then a (y', 87)-optimal estimator is a

y-optimal estimator

Proof Similarly to the proof of Lemma 8.6, let -ro E Me/l be such that the support of -ro is

contained in 87, xfo is a (y', 87)-optimal estimator, and 0* E 8 be such that supose R(8, ) =

R(0*,2). Then 0* E deo(co(A)) for some A E 2ae. Let 8' E 87 n deo(co(A)) be such that

| (0' 0*), | < 6(i). By the Taylor theorem, we have that for some 8" E 8 in the line segment

connecting 0* and 8'

R(0, ro)= (0*, fr) + ~(0' *)'"D2; (o(, / _

Certainly there exists a a2,. .. UN 6W Such that 8' = C By the h~ypoth~esis of th~e

theorem,

-(0' *)'D2R: p (0 l _0 0* pl _O O*TAo (0' *) sulp R(0l fr) i a suip R(0-, fr).

i= 1

Since a, (8' *): a,' < fi(i)2. Thus supose n(o:. ) -1P~ supos ro)

R(0*, ~ ~ ~ i= br)-R0,fo (i)2(4/2 supose R(8 ro). It can be verified that since

supose, R(8, fro) < V(87, K, R)(1 + y') and V(87, X9, R) < V(8, X9, R), then

supeoe R(8, fro) < V(8, X9, R)(1 + y). Thus fro is a y-optimal estimator. O

CHAPTER 9

THE RESTRICTED RISK BAYES PROBLEM AS A MINIMAX PROBLEM

Let v be an element of Me/l and Qo E [0, +oo]. In this section, we show that the problem

of finding a restricted risk Bayes solution relative to (v, Qo) is equivalent to a certain minimax

problem. The main result of this chapter, Theorem 9.1i, appears as a conjecture in [16].

We impose the following additional conditions in order to derive the results of this chapter:

Condition 9.1. There exists an essentially unique Bayes solution relative to v.

Let Qo = supoe, R(8, iv), where is denotes the (essentially unique) Bayes solution relative

to v.

Condition 9.2. V(8, .[. R) < Co < +oo.

Let RP(0, 2) = p r(v, 2) + (1 p)R(0, 2) and rP(-r, 2) = fe RP(0, 2)dr, where 0 < p < 1.

Let K(i, p) = supose RP(0, 2) and let K(p) = infe, g K(i, p). Since supose R(8, ~) > W, ~)

we have that

K(i, pi) > K(i, p2) 1f 2iIp. (9-1)

Lemma 9.1. Suppose Conditions 9.1 and 9.2 hold. Then K is concave, decreat~sing, and

continuous on [ 0, 1].

Proof See Appendix B. O

Let G(A) = supose R(8, i) rv ~).

Lemma 9.2. Let 0 < pi < p2 < 1. Suppose that it is a minimax estimator for (8, ./, RP1) and

2 is a minimal eStimatOTJOT (0, X P). Th8#

3) supose R8, ~1) < supose 2(8 *~

Proof See Appendix C. O

If 0 < p < 1, there is a strong relation between the estimation problems (8, K, R) and

(8, K, RP), which stems from the definition of RP.

Lemma 9.3. If fo is a Bayes solution relative to -r Me Zl in the estimation problem (8, X, RP),

then io is a Bayes solution relative to pv + (1 p)-r in the estimation problem (8, X, R).

Proof The lemma is an immediate consequence of the fact that for any is E rP(-r, 2)=

pr~~i)+ ( ~r~,2 = ~py+ ( ~r,).O

Remark 9.1. By Corollary if Conditions 4.1 and 4.2 hold for (8, K, RP), there exists a

minimax estimator in the estimation problem (8, X, RP). If in addition, 8 is compact and

Condition 4.3 holds, there exists a least favorable a priori distribution -ro E Me/l, and any

minimax estimator is a Bayes solution relative to -ro. A minimax estimator is an essentially unique

minimax estimator and is admissible if Condition 4.4 holds as well. In fact, it can be shown that

if 0 < p < 1, each one of Conditions 4.1-4.4 holds for (8 K RP) if it holds for (8 K R).

Remark 9.2. Certainly we can use the results of Section 7 for the estimation problem

(8, K, RP), where 0 < p < 1, provided that Conditions 7.1-7.2 hold for the estimation problem

(8, K, RP). In fact, if 8 is compact, it is sufficient that Condition 7.1-7.2 would hold for the

estimation problem (8, K, R). Indeed, suppose 8 is compact and Conditions 7.1-7.2 hold for

the estimation problem (8, X, R). It follows easily from Lemma 9.3 that for any -r Me Zl, there

exists an essentially unique Bayes solution relative to -r in the estimation problem (8, X, RP),

whence Condition 7.1 holds. Let if denote the (essentially unique) Bayes solution relative to -r

in the estimation problem (8, X, RP). Let {n)g, be a sequence in Me/l that converges weakly

to To E Me/l. Let -rf = pv + (1 p)-r for all i > 0. Then {-rf} converges weakly to -rd. Since

R(0, ~If) = R(0, 4) for all i > 0, R(0, ~If) converges to R(0, ~If) uniformly on the compact set

8. Since the convergence is uniform, r (v, if ) converges to r (v, ifo). Thus RP/C (0 f ) converges

to RP (0, ~If) uniformly on compact subsets of 8 and Condition 7.2 holds.

Lemma 9.4. Suppose X is subconvex and has the property (W*) and Conditions 9.1-9.2 hold.

Let Q(p) = -yp + P, where y < G(A,) and Pe R Suppose Q(p) > K(p) for all p e [0, 1].

Then if y > G(A) whenever & is a minimax estimator for (8, K, R), there exists i' E X such

that K (i', p) < Q (p) for all 0 < p < 1.

Proof Suppose y > G(A) whenever & is a minimax estimator for (8, X, R). Certainly Q is

continuous on [0, 1] and by Lemma 9.1, K is also continuous on [0, 1]. Thus there exists e > 0

such Q(p) K(p) > e for all p e [0, 1]. It is easy to verify that {K(i, -) : S E X(Co, R)} is an

equicontinuous family on [0, 1]. Thus there exists n > 1 such that

|K(p) K(p')| < e/2 and

|K~, ) -K~, p)|< e/2 whenever |p p'| < 1/n and is E (Co, R). (9-2)

Let pi = i/n and ~i be a minimax estimator for (8, X, R^i) for i = 0, 1, n. Note

that in = is and ~i exists for all 0 < i < n 1 by Theorem 4.1 and Remark 9.1. Since

io is a minimax estimator for (8, X, R) and in = is, G(in) > y > G(fo). Thus by

Lemma 9.2 part 1), there exists 1 < m < n such that G(im) > 7 > G(im-1). By Lemma

9.2 part 3), im-1 E X(Co, R). Let rl be such that rlG(im) + (1 rl)G(im-1) = 7. Let

K,7(p = q~im, ) + 1 -)K(:im-,_l p). Since K(i, p) = G()po + sulpose R(0, i), then

K,7l(p) = -7p + 17 supose R(H:0L, i) + (1 17) supose R(H, im-1)_l. It can be verified that

K(pm) < K,7(pm) = 17K(pm) + (1 rl)K(im-1, pm).) (9-3)

Using (9-2),

qK~p) +(1 q)Kim-, p) < K~p) +(1 q)Kpm-) +e/2 K~m) e.(9-4)

By (9-3) and (9-4), |K(pm,) K,,(p,)| < e. It follows K,,(pm) < 0(pm,). Since the line K,,(p)

is parallel to Q(p), we must have that K,7(p) < Q(p) for all 0 < p < 1. Since K` is subconvex,

there exists i' E X such that supose R(0, i') < rl supose R(8, im) + (1 rl) supose R(8, im-1)

and r (v, i') < l r (v, im) + (1 -q) r(v?, im-,)l. Therefore, K (.i', p) < K~, (p) < Q (p) for all

0 < <1. O

Theorem 9.1. Suppose X is subconvex and has the property (W*), Conditions 9.1-9.2 hold,

and V(O. .F. R) < Co < Co. Then io E X is a restricted risk Bayes solution relative to (v, Co)

if and only if there exists 0 < po < 1 such that supose RPo (8, f o) = inf e,x supose RPo (0, 2) and

supose R(8, fo) = Co.

Proof Suppose there exists 0 < po < 1 and io E X such that suposeRP(8 ro 9)

inf~Ex supose RPe(0, 2) and supose R(8, fo) = Co. Suppose that io is not a restricted risk Bayes

solution1,+ ,, re at v to (v C o) T hen, there ,:,,, exists, ` an e t m t r i' such that su pose R (,E'

Co and r (v, i') < r (v, fo). This implies that supose RP0o(0, i') < supose RP0o(8, fo), which is a

contraiction.IUI HeceICC 1o is a re;strictdU 15 risk C; Bayes 1I souionrlaiZLve to (v, C/o).

Suppose that io is a restricted risk Bayes solution relative to (v, Co). LCertainly supoeO IL(0,O 1o

Co.Supoe upseR(,i) C. etQr) pr(v, fo) + (1 p)~Co. Then Q(p) > K(io, p)

for all 0 < p < 1 and Q(1) = K(io, 1). Since Co < Co, So is not a Bayes solution and

K((io, 1) > K(1). Thus Q(p) > Krp for Iv all 0 < pv < 1. Let = Co r(v, fo). Then y < G(is)

and Q(p) = -yp + Co. Let i* be a minimax estimator for (0. .F. R). Since io is a restricted risk

Bays sluionreltie t (v C) ad C >V(8, K, R), r(v, fo) < r(v, i*). Thus y > G(i*).

By Lemma 9.4, there exists i' E X such that K(i', p) < Q(p) for all 0 < p < 1. It follows

thatZ supeoe R(0\, LI ') < o and r(v, i') < r(v, fo). This contradicts the fact that io is a restricted

risk, Bayes,..:, solution reatv to, (v o) Thus~ suoeR(,,) o Suppose there exists no

0 < p < 1 such that supose RP(8, fo) = inf~Ec supose RP(0, 2). Then K(io, p) > K(p) for

all 0 < p < 1. Since supose R(8, fo) = Co, K(io, p) = Q(p) and by Lemma 9.4, there exists

i' such that supose R(8, i') < Co and r(v, i') < r(v, fo), which contradicts the fact that io is

a restricted risk Bayes solution relative to (v, Co). Therefore,, there exists 0 < p~o< 1 suc thatl IIL

supose RPe(0, fo) = infc,x supose RPe(0, 2). Finally, since V(8, K, R) < Co < Co. O

Under the hypothesis of Theorem 9.1i, finding a restricted risk Bayes solution relative to

/, (v:,.:,,,, Co) s euivlentto indng minimax, esimto for,+:,+, the~1, estmaio prole (8, C, R o

some 0 < p < 1.

Lemma 9.5. Suppose X is subconvex and has the property (W*) and Conditions 9.1-9.2

hold. Let {p,}g, be a sequence in [0, 1] that converges to po E (0, 1). Let ~i be a minimax

estimator for (8, K RPi) for i = 0, 1, .. Then if V (8, K R) < supose R(8, f o) < Co,

supeoe R(8, fi) supose R(8, fo).

Proof Let M~ = supose R(8, fo) and suppose V(8, K, R) < M~ < Co. Fix e > 0 such that

M~ e > V(8, K, R) and M~ + e < Co. By Theorem 4.2, there exist restricted risk Bayes

solutions relative to (v, M~ e) and (v, M~ + e). Therefore, Theorem 9.1 yields p' and p" in (0, 1)

such that supose R(0, *, ) = M~ e and supose R(0, x*,,) = M~ + e, where i*r is a minimax

estimator for (8, X, RP) for all p E (0, 1). By Lemma 9.2 part 3), p' < po < p". In fact, since

supeoe R(0, *,) < supose R(8, fo) < supose R(0, *,,), we must have p' < po < p". Thus

p' < pi < p" for i sufficiently large. Lemma 9.2 part 3) furnishes that | supose R(8, ii) M|1 < e

for i sufficiently large. It follows that supose R(8, ii) supose R(8, fo) O

Remark 9.3. If X is subconvex and has the property (W*), Conditions 9.1-9.2 hold, and

V(8, L, R) < Co < Co, a restricted risk Bayes solution relative to (v, Co) can be solved

in the following manner: Find po E (0, 1) such that supose R(8, fo) = Co, where io is a

minimax estimator for (8, X, RPO). The estimator io is a restricted risk Bayes solution relative

to (v Co). If a minimax estimator for (8, K RP) can be found and the supremum of its risk

can be calculated for all p e (0, 1), Theorem 9.1 and Lemma 9.2 part 3) imply that we can

find a sequence {pi } that converges to po. By Lemma 9.2 part 3), such a sequence can be found

easily using the bisection method. 1Moreover Lemma 9.5 implies that if {ps} is a sequence that

converges to po and ~i is a minimax estimator for (8, K, RPi), then supose R(8, in) is close

to Co whenever p, is sufficiently close to po. Thus for a sufficiently large, in can be used to

approximate a restricted risk Bayes solution relative to (v, Co) with any desired accuracy. Thus

we have a practical way to derive a restricted risk Bayes solution relative to (v, Co).

CHAPTER 10

ESTIMATION WITH A RESTRICTION ON THE OBSERVATIONS THAT CAN BE USED

As before suppose we are given a probability space (R, -T2, Q), where R = Y x X and

-T& = FTy x FTx, and random elements Y : R H (f FTy) and X : R H (X, FTx), which are

jointly distributed and their distribution Q is an element in a class {Po : 0 E 8} of measures

on (R, -T2). So far we considered the case that the experimenter observes an observation

ye E the value taken by Y, and based on this observation estimates the parameter x, which

is the true value of X; little was assumed on the structure of the spaces y and X. Let S, and

S, denote arbitrary sets. Suppose for each A E So there is associated a measurable space

(3x FTx ) and for each A E S, there is associated a measurable space (y x, Fy,). In addition,

suppose X = n,, s, y >es,~ Yx, E = nes. Ex,, and Fy = ns,~ FT>. Let

Y, : R (P x, FTy ) and Xx : R H (3x FTxx) be the random elements defined as follows: For

each we E Y (w) and X ( ) are the projections of Y(w) and X(w) into the spaces yi and 3x ,

respectively. Let yx and xx denote the true values of Yx and X respectively. Given a subset S

of S,, let Tis = nxas yi and FTys = n,,s -Ty,. Let Ys : OR (Ps, -Tys) denote the random

element defined as follows: For each Lce E Ys(Lo) is the projection of Y(Lc) into the space Ps.~

Let y7s denote the true value of Ys.

In many applications there are restrictions on the components of y that may be used in order

to estimate components of x.

Definition 23. Given the sets S, and S, defined above and a mapping & : S, 2sy, the triplet

(S,, S,, h) is called an estimation space. An estimator for x \subin t to (S,, S,, h) is a collection

{ix : A E S, }, where &x is an estimator for xx based on Yh( ).

Thus an estimation space completely specifies which components of the parameter x are

to be estimated and which components of the observation y can be used. Suppose (S,, S,, h) is

an estimation space and for each A E So there is associated a space D the space of possible

estimates for x a space Xx, the space of available estimators for xx based on Yh( ), and a risk

function Rx : 8 xA Rxi U {+oo}. In what follows we assume Rx is nonnegative for each

A E S,. Then in fact there are given a collection of estimation problems {(8, X9x, R ) : A E S,}.

Fix A E S, and S C S,. By an estimator &x of xx based on ys, we mean a mapping from Pis

into -rD, such that ys H x(D |ys) is FTys-measurable for all DI ae BD). Let D = ness Dr

and X = n,,s. X Then D and X are the space of possible estimates for x and the space of

available estimators for x, respectively.

If, in addition, there is given a (nonnegative) risk function R : 8 x .9 R WU {+oo}, the

problem is similar to the problem treated so far. Given & : S, 2sy, for each A E S,, let PoX

denote the joint distribution of Yh,3 and X when 8 is the true state of nature. If S is a subset

of S,, we let Po~s denote the marginal distribution of Ys, when H is the true state of nature. If

Ae S we let Pux^ denote the marginal distribution of X when O is the true state of nature.

Given a class A of estimators for x and A E S,, we use Ax to denote {ix : f E A}.

It is left to specify a risk function for (8, X9). One possibility is R(0, 2) = sup,,ss R (0, ~x).

If for each A E S,, there exists a minimax estimator for x which is denoted by if then

i* = { 1 : A E S,} is a minimax estimator for x. Indeed, it is easy to see that

inf sup R(0, 2) inf sup sup R (0, & ). (10-1)

Since for each &x E K supeoe R (0, & ) > supose Rx N, 1r),

inf~ sup sup R (0, & ) > sup sup R (0, if ) sup R(0, i*). (10-2)

By (10-1) and (10-2), i* is a minimax estimator for x. Thus with the above risk function,

minimax estimation for x subject to (S,, S,, h) is completely determined by minimax estimation

for xx (A E S,).

Let us consider another possibility for the risk function. Suppose S, C N. Let w : S,

[0 + 00). We can define the risk function

Without loss of generality, we may assume that w(A) > 0 for each A E S, since if w(A) = 0 for a

certain X, we can remove that A from S, without affecting the risk function R.

Theorem 10.1. Suppose there are given a collection of estimation problems {(8, X R ) : X E

S, }, where S, C N. Let K = H Ass. K and let R(0, 2) = C,,es R (0, & ) w(A), w (A) > 0

for each A E S,. Then

1. If Conditions 4.1 and 4.2 hold for (8, K R ) for each A E S,, then Conditions 4.1 and

4.2 hold for (8, K, R).

2. Suppose S, is finite. Then if Conditions 4.3 and 4.4 hold for (8, K R ) for each A E Sz,

then Conditions 4.3 and 4.4 hold for (8, K, R).

Proof 1) Fix i', i" E .9 and 0 < a~ < 1. Then for each A E S,, there exists if; aKX such

that R (-, x)

R(-, i*) < a~R (-, ') + (1 a~)R (-, i"), whence X is subconvex. Thus Condition 4. 1 holds for

(8, X, R).

For each A E S, let 9* be a compact metrizab~le space and RIafnto fo

into [0, +oo] such that (K R ) ~ ( X*, Rr;), Rr (0, -) is lower semicontinuous on X* for each

Se 8 and Rr;(-, a) E m(B(8)) for each as E *. Let X* = nes. X* be equipped with the

product topology. Let R*(0, 2) = C,,es Rr;(0, & )w(A). Then by Tychonoff's Theorem, 9 '

is compact. In addition, X* is metrizable since it is a countable product of metrizable spaces.

Since for each A E S, andu v e 2 RIL(0, & ) is lower semicontinuous, for each finite subset

S of S,, CExs Rr;(0, & )w(A) is lower semicontinuous. Since the pointwise supremum of

any collection of lower semicontinuous function is a lower semicontinuous function, we have that

R*(0, -) is lower semicontinuous on X* for each 8 E 8. Since the pointwise limit of a sequence

of measurable functions is a measurable function, it is easy to show that R*(-,~x E ) m(B(8)).

It is also rather straightforward to show that (X9, R) ~ (X*, R*). Thus Condition 4.2 holds for

(8, X, R).

2) The proof is rather straightforward and is omitted. O

In the case of countable S,, Theorem 10.1 implies that under rather weak conditions on

(8, K R ) (A E S,), there exists a minimax estimator for (8, X, R) and a restricted risk

Bayes solution relative to (-r, Qo) for each -r Me Zl and Qo > V(8, K, R). The theorem is

especially useful for the case of finite S, since it specifies that if certain conditions holds for each

one of the estimation problems (8, K R ) (A E S,), they hold for (8, .[, R) and hence the

results of the previous chapters are valid for (8, .[. R).

Example 10.1. Suppose the experimenter observes a sequence {y, } of observations, where

y, E R">Y, and there are given a sequence {x, } of parameters, where x, E RWN. In ;///1 case,

Ai = RWN and it is convenient to take FTy, to be B(Ai). Suppose there is given an estimation

space (S,, S,, h), where S, = N, S, C N, and & : S, 2s. The observation y is \imphl, the

sequence {y, }. For example, if S, = N and & : n E S, H {0,. ., n}, we have the so-called

casual filtering problem (assuming the index n is a time index). If h : n e S, H {0,. ., a 1},

we have the so-called one-step prediction problem. If at is a positive integer S, = {0, 1,. ., at },

and & : n E S, H {0,. ., n}, we also have a certain casual filtering problem. Also,

estimation of a continuous time process can be entered to dIri\ formulation. Put S, = [0, +oo),

S, C [ 0, +oo) and, for example, let & : a s [ 0, +oo) [ 0, a]. In ://i\ case the observation y is a

function (or a signal).

Example 10.2. Consider the following discrete-time linear stochastic system in state-space form:

Xn I = Fox, + In~,,, a > 0,

yn = He x, +v, ,

where x, E RWN" (n > 0) is the state vector y, E RWN is the system output, v, E R"~

is the measurement noise, In~,, E RWN" is the model noise, and H, and F, are matrices in

RWNXN and RWNXN, respectively. Suppose there is given an estimation space (S,, S,, h),

where S, C N, S, = N, and for each A E S,, h(A) is a finite subset of S,. This example

is certainly a special case of the previous example. Note that the filic ,rio. prediction, and

\rl ur llrings problems are all special cases of dIri\ example. In the most general case, there are

stochastic and/or deterministic uncertainties in xo, O',1, {va }, {H, }, and {F, }. For example,

in some problems the initial state vector xo can be modeled as a deterministic and unknown

vector which is known to belong to a set 31o C RAN. This is a deterministic uncertainty. Of

course, in other problems xo may be modeled as a random vector whose distribution is known

to belong to a certain class of distributions (e.g., the Gaussian distribution with zero mean and

some restriction on the covariance). This is an example of a stochastic uncertainty. The system

noise sequence {tr~,, }, for example, can be modeled as a random process whose distribution

is known to belong to a certain set (e.g., a subset of the set of joint Gaussian distributions of

a sequence of independent random vectors), but in some problems can be better modeled as

a deterministic sequence that is known to belong to a certain set (e.g., a subset of the set of

bounded sequences). The relations between the quantities xo, O',1, }, {va }, {H, }, and {F, } are

also important. For example, suppose xo, {tr~,, }, and {v, } are modeled as random with stochastic

uncertainties, then the dependence between l;

also be modeled. The most convenient way to spec~if\ 8, in ;//i\ general case, is it to assume

that (xo, ( I,,, }, (vn}, {H,}, {F,}) is a value of a random element whose joint distribution

belong to a class 8 of measures defined on the appropriate product space. There is no problem

with dIri\ formulation since even if one of l;

regarded as random with distribution that assigns probability 1 to a certain value in No0. Our

basic assumption regarding 8 was that 8 is a metric space. This is true if 8 is equipped with

the topology of weak convergence since xo, I,',,- vn, H,, and F, (n = 0, 1, ..) are defined on

separable metric spaces. Thus the results of previous chapters and dIri\ chapter may be used.

Note that in dIri\ case an element of M/e is a probability measure defined on a class of probability

measures. Luckily, in many important cases, we can use instead of the set 8 defined above a

simpler equivalence class, as illustrated in the following example.

Example 10.3. Suppose that in the previous example { H,} and { F,} are known sequences,

the sequences {v, } and {tr~,, } are uncorrelated, v, and vm are uncorrelated for n / m, and

In~,, and I,. are uncorrelated for n / m. In addition, suppose the initial state xo and v, (Ir,, )

are uncorrelated for all n > 0. Suppose also that the distributions of In~,, (v,) and I,. (vm)

are identical for each n and m and have zero mean. As in the previous example, suppose h(A)

is a finite subset of S, for each A E S,. Suppose xo is the value of a random vector whose

disvtr-ibtion belongs to a set 81 o~f measures. on (RA, B(RN.)), vu, is thep value of a randonm

vector whose distribution belongs to a set 82 Of meaSuTes On (WN aINy )) (n = 0, 1, ..),

and In~,, is the value of a random vector whose distribution belongs to a set 83 Of meaSuTes On

(RAN, B(RWN,)) (n = 0, 1, ..). Let 8 = 81 x 82 x 03, where ife Oi 4for i = 1, 2, 3,

O1x 02 x 3 is the product measure. Then 8 can be regarded as the space of state of nature

instead of the space used in the previous example. Suppose R(0, 2) = C,,es R (0, & )w(A),

R (0, & ) = fygX) xxh SDX L (Zx d )di:($ ?i(d |,3dr (y, x), and L (xl d) = | V(x d)|2

where Vx RN x N"XN. If~ in addition, we assume that each measure in 8 is Gaussian, the space

of states of nature can be further \imlrrllfie d. Indeed, for each 8 = Or x 02 x 83 E O, Ele i 8e

and As (0) denote the mean vector and autocorrelation matrix of Os, respectively, for i = 1, 2, 3.

For examley6, if 0 = Or x 02 X ~83 thena 11 (0) = fx ,.."'i rand At (0) = fx~o xox dO LePt

oi = {((Di(e), Ag(e)) : e 8 } for i = 1, 2, 3. Let C7 = RWN x ga, C2 IN x N, and

C73 INze xNzc Le 1~ 2 C<* ~ C~3 eranly i S a vector space with coordinate-wise

addition and multiplication by a scalar (i = 1, 2, 3). Given as WE let | |a| |4e = |a(1) | + | |a(2)1 | |2

Similarly, C7 is a vector space with coordinate-wise addition and multiplication by a scalau:

Givenz as E let ||a||v = C:= ||a(j)||4-~. Let 8 = 81 x 82 x O3. Th8# O is a subset of W.

The set 8 equipped with the norm |I | | | is clearly a metric space. Then it is more convenient to

regard 8, which is a subset of finite dimensional normed space, as the space of states of nature

instead of 8 provided that P is a Gaussian family. Note that if the class of estimators is restricted

to affine estimators, since the loss function Li is quadratic for each i E S,, then II idust~, loss of

generality, we may assume that each measure with bounded covariance in 8 is Gaussian since it

can verified that the risk function depends only on the mean vector and autocorrelation matrix of

dur \le probability measures.

In the rest of this work, we apply the general theory of previous chapters to the state

estimation problem of Example 10.3.

CHAPTER 11

THE STATE ESTIMATION PROBLEM

In this chapter we consider in more detail the state estimation problem of Example 10.3.

For the sake of clarity, let us repeat the formulation of this problem. We consider the following

discrete-time linear stochastic system in state-space form:

X,+1 = Fox, + It',,, a > 0,

Yn = H,x, + v., (11-1)

where x, E RWN" (n > 0) is the state vector, y, E RW~y is the system output, v, E RW~y is

the measurement noise, It',, E RWN" is the model noise, and H, and F, are matrices in RWNyXN

and RWN, X respectively. We assume that there is given an estimation space (S,, S,, h),

where S, C N, S, = N, and for each A E S,, h(A) is a finite subset of S,. We assume that

{H,} and {F,} are known sequences, the sequences {v,} and {tt',,} are uncorrelated, v, and

vm are uncorrelated for n / m, and it',, and w., are uncorrelated for n / m. In addition,

the initial state xo and v, (I,',,) are uncorrelated for all n > 0. We assume xo is a Gaussian

random vector and {v,} ({tt',,}) is a sequence of identical Gaussian random vectors. We

assume that the mean and covariance of xo and the covariances of v, and I,',, are unknown; it

is only known that each one of these quantities belong to a certain set. The space of states of

nature 8, in this case, is 81 x 82 x 02, where 81 is the class of possible mean vector and

autocorrelation matrix pairs for xo, 02 iS the class of possible mean vector and autocorrelation

matrix pairs for v, (n = 1, 2, ...), and 83 iS the class of possible mean vector and autocorrelation

matrix pairs for It',, (n = 1, 2, .. .). As mentioned in Example 10.3, the set 8 is a subset of

a finite dimensional space normed space C7 (see the definition of C7 in Example 10.3). For

8 = Or x 02 x 83, where 04 E 84 for i = 1, 2, 3, we let ri(0) and As(0) denote the mean

vector and autocorrelation matrix of Os, respectively, for i = 1, 2, 3. Our main assumption

regarding the space 8 is that A2(0) > 0 for each 8 E 8. We consider the risk function

R(0, 2) = C,,es R (0, & )w(A) for this problem, where w(A) > 0 for each A E Sz,

and Vx R IN"XN (the reader is referred to previous chapters for the above notation as well as

subsequent notation). Note that P is a Gaussian family of distributions.

Suppose V(8, ~D, R) < +oo and V(8, L, R) < +oo. When 8 is bounded and S, is finite, it

is obvious that V(8, ~D, R) < +oo and V(8, L, R) < +oo. In the case that S, is finite and 8 is

not compact or the case that S, is not finite, it is not necessarily the case that V(8, ~D, R) < +oo

and V(8, L, R) < +oo. It is outside the scope of this work to derive the exact conditions for

V(8, ~D, R) < +oo and V(8, L, R) < +oo since we are mainly interested in the case that S,

is finite and 8 is compact. It is sufficient to mention that these conditions will depend on system

theoretic notions such as constructibility, stabilizability, and detectability. The interested reader

is referred to [24], where such conditions are derived for the special case of uncertainties in the

initial state.

Let us show that Conditions 4.1, 4.2, and 4.4 hold for the estimation problems (8, ~D R )

and (8, L R ) for each Ae E and that Condition 4.3 holds as well if 8 is compact. Fix

Ae E Our first step is to show that Conditions 5.1-5.7 hold. Since the loss function Lx is

quadratic, it is only left to show that Conditions 5.2 and 5.4 hold (Chapter 6). Consider the

conditional distribution of Yh( ) given X = x, when 8 is the true state of nature. Let Po^(-|IX = x)

denote this conditional distribution, which is certainly Gaussian. Moreover, since A2(0) > 0 for

e~tac r a 8, thefamly,~x~ { o ( X= ) : 0 E 8, X EX } is dominated by the Lebes gue-B orel

distribution of Yh( ) given Xx = x when 8 is the true state of nature. Let Ae B (i( 3)) be such

that p l(A) = 0. Then Po^(A|X = x) = 0 for each x E X. It can be verified that this implies that

Po^(A|X = x ) = for each xx EX 3. Thus~ then family {P (-X~ =( x ) : 0 E 8, XX E3X }

is dominated by pr and Condition 5.2 holds. Put Z: = [Y~, X~ ]. L~et {0,}) be a sequence in 8

that converges to 8o E 8. Then rli(0,) i qi(80) and As(0,) A s(00), for i = 1, 2, 3. It can

be verified that this implies that Eo, (Z) Eeo (Z) and Eo, (ZZT) Eeo (ZZT). It follows

that Po\, converges weakly to Po>,. Since the sequence {0,} is arbitrary, {Po\}) converges weakly

to PoX whenever {0,t} converges to 8o, {O,z} E 8, and 8o E 8. It follows that 8 Po^(A) is

B(8)-measurable for each Ae B (Pihz)) x B(3x ) [40].Thus Condition 5.4 holds. It is also

clear that 8 H Ea(Z) and 8 H Ea(ZZT) are continuous on 8. It now follows from the results

of Chapter 6 that Conditions 4.1, 4.2, and 4.4 hold for (8, ~D R ) and (8, L R ) for each

A E S,., and Condition 4.3 holds as well if 8 is compact. By Theorem 10.1i, whether (8, ~D, R)

or (8, L, R) is considered, there exists a minimax estimator, and there exists a restricted risk

Bayes solution relative to (-r, Co) for each -r Me Zl and Co > V(8, ~D, R). Suppose in addition

that S,. is finite and 8 is compact. Then we have the following results for the estimation problem

(8, ~D, R): (8, ~D, R) is strictly determined, there exists a least favorable a priori distribution

I-o E Me/l and a conditional mean estimator relative to -ro is an essentially unique admissible

minimax estimator. Moreover, the class of conditional mean estimators is essentially complete.

Note that, in general, a conditional mean estimator relative to -r is not a LMMSE estimator since

-r may not assign mass 1 to a single point in 8. Similarly, we have the following results for the

estimation problem (8, L, R): (8, L, R) is strictly determined, there exists a least favorable

a priori distribution -ro E Me/l and a LMMSE estimator relative to -ro is an essentially unique

admissible minimax estimator. Consider the filtering problem. Then a LMMSE estimator relative

to -r is not necessarily a KF if -r does not assign mass 1 to a single point in 8. There is a special

and important case in which a LMMSE estimator with respect to -r M y/1 is a KF. We will treat

this case in the sequel.

In the following chapters we consider two special cases of the above problem. The first case

is the case of stochastic uncertainties in the initial state, model noise, and observation noise with

L as the class of available estimators. The second case is the case of deterministic uncertainties in

the initial state with ~D as the class of available estimators. In both cases we will assume that S,. is

finite and 8 is compact. Note that by Remark 4.1, the case that 8 is bounded, but not necessarily

compact is also covered. These two cases are important on their own merit, and they will also be

used to illustrate some of the general results of previous chapters. For example, the first case will

be used to illustrate the method proposed in Chapter 9 to derive a restricted risk Bayes solution

(Remark 9.3) and the second case will be used to illustrate the method proposed in Chapter 8 to

derive an approximation for minimax estimators.

CHAPTER 12

AFFINE STATE ESTIMATION BASED ON QUADRATIC LOSS FUNCTIONS

In this chapter we consider a special case of the state estimation problem of Chapter 11. We

consider the case of state estimation with stochastic uncertainties in the initial state, model noise,

and observation noise with the class of available estimators being the class of affine estimators.

Throughout this chapter, we assume 8 is compact. Thus V(8, L, R) < +oo. In this chapter,

we assume that the estimation space (S,, S,, h) is such that S, = {0, 1,..., nt}, S, = N, and

h(k) = {0, 1,. ., Gk } for each k E S,, where at and nk, for k = 0, 1,. ., nt, are nonnegative

integers. We assume that w(k) = 1 for each k E S,, i.e., R(0, 2) = C"' Rk(8, k~). Given

Co > V(8, L, R) and ve 8 our goal is to find a restricted risk Bayes solution relative to

(v, Co).

12.1 Finding a Restricted Risk Bayes Solution

First, we want to derive a closed form expression for R(0, i). Let F,, = FF _1 Fj (i >

j) and Fi~i = Fi. We will use the convention Fi~i 1 = I and Fi,y = 0 if j > i + 1. Let

0,, = H' Fo To, FoH2T n T-1,0Hlf]".Let ( = [0 HJ 1 FH2T nT-1,1HTI] =

[0 1 0, H2 F2HT'""FT1, 2HI 000H3 -,H],.. 00 I

Let I, = [1, 1, 1"]. In addition, Let I( = 0 for 1 > n. Let y" = [yoT yT yT]. Note that

Yh(k) = Unr It is easy to verify that

y"n = OnXO + E n I- + u, (12-1)

Zk Fk-,0,z 0 P -1it -1, (12-2)

i=1

where w" = [wo" wT -- (] and u = [VTo VT -- Let k, > 0 and n > 0. By (12-1) and

(12-2),

nVk-1

i=0

For each k E S, and Zk, = Ayo" + be E k, let As, = A and be, = b. Let Wk, = VkT ~, for

k = 1, .., us. By our assumptions regarding the noises and the initial state,

Rk8,k)= tr(VW(As 0,, A-1,_lo)A(0)(As 0,n FI-i,o) VkT>

nkVk-1]

i=0

+2~ 71/ ~b, WkI(As 0,~ F-1,o)917(e)+ |Kb,|2.12 (12-3)

Let r1(0) = [r1(0)T ra2) rl3(0) ]T and let A(0) be the block diagonal matrix with Az(0),

A2(0), and A3(0) in its diagonal blocks. Let Ay,4(0) = As(0) ri(0)ri(0)T for i = 1, 2, 3.

We assume that some a priori information regarding the true state of nature 8 E 8 is

available. The case that no a priori information is available is an important special case. The a

priori information is given in the form of a nominal v E 8. For simplicity, we assume rll(v) = 0.

There is no loss of generality in this assumption since if rll(v) / 0, we can translate the states

and observations and bring the problem to this form. Below, we summarize the assumptions

taken so far together with some new assumptions.

Assumption 12.1. ve 8 and rl(v) = 0.

Assumption 12.2. 81 is a compact and convex subset of~ and (-rl, A) E 81 whenever

(rl, A) E 81.

Assumption 12.3. 82 is a compact and convex subset of % and for all 8 E 8, rl2 (0) = 0 and

A2 (0) > 0.

Assumption 12.4. 83 is a compact and convex subset of % and r/3(0) = 0 for all 8 E 8.

The assumptions that 84 is compact and convex for i = 1, 2, 3 can be somewhat relaxed,

but, for the sake of clarity, it is advantageous to make these assumptions. The assumption that

(-rl, A) E 81 whenever (rl, A) E 01 is rather weak and is satisfied in many important cases. This

assumption is clearly satisfied in the case that 81 = {((9, Al + rlrl) : rl E FI, Al E 02}, where

01 = {9l : (rl, A) E 01} and 02 = {A rlrl : (rl, A) E 01}, and El is symmetric around the

point 0, i.e., rl E FI if and only if -rl E Fr. It is assumed that Assumptions 12.1- 12.4 hold in the

sequel. Note that 8 is then a compact convex subset of C?.

By (12-3), Rk k~) is both convex and concave on 8 for all Ak E k~, i.e., for any

0 < a~ < 1 and 8', 0"1 E 8,

In addition, Rk ', k) is COntinuous on 8 for all ik E k~. It follows that R(-, 2) is both convex

and concave on 8 and continuous on 8 for all i E L. Let r(-r, 2) = fe R(0, 2)dvr and let

Tk-r To ) = So k 8, k~)dTr for k = 0, 1,. ., us. Let Z be the class of all finite subsets of 8. Let

Me/l denote the space of distributions in Me/l with finite support. Let -r Me Zl. Then there exists

Z E Z and 81,...,0z2 E 8 such that Z= {01,...,0z }) and r(Z) = 1. In this case,1let r(i)

denote the mass that -r assigns to the point Of f~or i = 1,...,1 |Z. Finally, let H o r = C1 Osir(i).

Since 8 is convex, 8 o re 8 Since R(-, 2) is both convex and concave on 8 for all ie E ,

Lemma 12.1. Let -r* E Me/l. Then there exists 0* E 8 such that r(-r*, 2) = R(0*,i) for all

is L As a consequence, & is a Bayes solution relative to -r* if and only if & is a Bayes solution

relative to 0*. Similarly, ri(-r*, is) = Ri (0*, ~i) for all ~i E 4 and ~i a Bayes solution relative to

-r* if and only if is is a Bayes solution relative to 0* (i E S,).

Proof Fix is E Since 8 is compact, it is separable. Thus the space of distributions with finite

support, Me~l, is dense in Me/l in the sense of weak convergence [27, Appendix 3]. Let {74}) be a

sequence of distributions in Me/l that converges weakly to -r*. Since R(-, 2) is continuous on the

compact set 8, R(-, 2) is bounded and

/ R(0,,, 2)r im (0 )do (12-4)

Let Of = 0 o nr (i = 1, 2, ..). Since 8 is compact, there exists a 0* E 8 and a sub sequence {Of f

such that {0fm } converges to 0*. Since R(-, ) is continuous on 8,

R(0*, 2)= =limn R(Ofm ). (12-5)

Since R(-, 2) is both convex and concave on 8 and -r has finite support,

/ R(, 2du R(O ) ( = 1 2,. ..).(12-6)

By (12-4)-(12-6), r(-r*, 2) = R(0*, 2). The proof follows from the arbitrariness of 2. O

It is clear from the proof of Lemma 12.1 that if -r* e Me/, is a Bayes solution relative to

-r* if and only if & is a Bayes solution relative to 0*, where 0* = 0 0 -r*.

We want to apply the results of Chapter 9 for the estimation problem (8, L, R). We have

already shown that Conditions 4.1-4.4 hold in Chapter 11. Let us show that Conditions 7.1, 7.2,

and 9. 1 hold as well. In Chapter 11 it was shown that L has the property (W*) and Condition 4.4

holds. Thus Theorem 4.2 implies that Condition 7.1 holds. Certainly the weaker Condition 9. 1

must hold as well. It is left to prove that Condition 7.2 holds for (8, L, R). In fact, it is sufficient

to prove that Condition 7.2 holds for (8, k, k~) for each k E S,. Fix k E S,. For the sake

of clarity, we prove that Condition 7.2 holds for (8, k, k~) in the case that r11(0) = 0 for all

8 E 8. It is easy to verify that this is true also in the more general case of Assumption 12.2 but

the expressions are rather cumbersome. Assuming r11(0) = 0 for all 0 E 8 and using (12-3),

Rk 8 ~k) = tr.( /Aiyk,/1(0)A Vk ) 2 tr.( /~A~iY k,2(0) VkT)

+ tr( 7kI,3(0)Vk')+ + |Kbyl |2, (12-7)

where

7k,1 n) Az Ch(0)O(, + I,,, O A2(H ,~, y 1 A3(),

nlkAk

7k,2(B k-1,0Az(0)O(~B + Fk-1,iA3)~k)

i= 1

Recall that ||2'k Ok~ = ||A. A. || I I. b~I ~., LII~ iS a norm, making k

into a normed space.

Let it,k E k~ denote the (essentially unique) Bayes solution relative to -r Me Zl. It is not

difficult to show that

Aenr = Yk,2(r)7. Y,(7) and ban~ = 0, 128

where we extend yk~i from 8 to Me/l by defining yk~i 7r) = yk,i 8 o Tr) for all Te ME Zl (i = 1, 2).

Let {-ri} be a sequence in Me/l that converges weakly to -ro E Me/l. By Lemma 12.1i, there

exists a sequence {Os} E 8 and an element 8o E 8 such that Rk(8, 74,k) k R(8, Os,k)

(i = 0, 1, ..). Certainly yk~i is COntinuous on 8 for i = 1, 2. It follows easily from (12-8)

that the mapping 8 H O,k is COntinuous on the compact set 8 and hence uniformly continuous

and bounded. Thus there exists p > 0 such that SO,k E Bp for all 8 E 8, where B, = {ik E

Ck : k1~1~ I p}. It is easy to see that RkIS isCOntinuous on the compact set 8 x B,. Thus

{Rk~(0, -) : 0 E 8} is equicontinuous on B,. Since 8 H 20,k is UnifOrmly continuous on 8, the

family {0 Rk (8 8,~) : 8' E 8} is equicontinuous on 8 and therefore Rk (8, 0s,k~) COnverges

to R, (8, Bo,k~) UnifOrmly on 8. It follows that R, (8, 74,k) COnverges to Rk(8, To,k) UnifOrmly on

8 and Condition 7.2 holds.

Let 2, denote the (essentially unique) Bayes solution relative to -r Me Zl in the estimation

problem (8, L, R). Since 8 is compact and R(-, 2,) is continuous on 8, Co < +oo and

Condition 9.2 holds if V(8, L, R) < Co. Let AP = Ap, +(lp)r. By Lemma 9.3, AP is a Bayes

solution relative to -r in the estimation problem (8, L, RP). Let rp(,, 12) = pr(v, 2) + (1 -

p)r(-r, 2). Let r"P(-) = rP(,, AP). Since Conditions 4.2-4.4, 7.1, and 7.2 hold for the estimation

problem (8, L, R), they hold for the estimation problem (8, L, RP) for 0 < p < 1 (Remarks

9.1 and 9.2). Thus using the results of the previous chapters and Lemma 12.1i, if 0 < p < 1,

there exists 8o E 8 such that r"P(8o) = supoe, r"P(0) and "0 is an admissible, essentially unique

minimax estimator. In particular, by setting p = 0, there exists a minimax estimator for (8, L, R).

Let us consider the problem of finding a minimax estimator for the estimation problem

(8, L, RP), where p E [0, 1). As mentioned earlier, if V(8, L, R) < Co < Co, the solution of

this problem for p e (0, 1) is necessary in order to find a restricted risk Bayes solution relative

to (v, Co) using the method of Remark 9.3. The case p = 0, corresponds to minimax estimation.

Note that if Co > Co, the problem is reduced to regular Bayes estimation and is is the solution.

The case Co = V(8, L, R) corresponds to minimax estimation.

Let 0o = {0 E 8 : r1(0) = 0}. Let us show that 0o is a convex and compact subset of

8. Fix 0 and 8' in 80 and 0 < a~ < 1. Let On = caO + (1 a~)0'. Since 8 is convex, On E O.

Clearly r1(8a) = 0. Thus On E 0o and 80 is convex. Let {Os} be a sequence in 0o that converges

to 8o E 8. Then by Assumptions 12.2-12.4, r1(8o) = 0. Thus 80 is closed. Since 8 is compact,

80 is compact. Let to = {ie E : by, = 0 for k = 0, 1,..., nt}.

Lemma 12.2. Let 0 < p < 1 and consider the estimation problem (8, L, RP).

Consider the following algorithm:

Step 1: Choose 01 E 80 and let i = 1.

Step 2: Findl Of a 80 sucrh that R(Of i ) = supoeaO R(8, if ).

Step~ ~ ~~~V 3: IfR(s i )=R(f, ) then stop; the distribution Os is a least favorable a priori

distribution.

Step 4: Let 80,i = cli + (1 a)04 for asE [0, 1]. Find to E [0, 1] such that r"P(0a,,i)

supoe [o, 1] TP(H0,i).

Step 5: Put 8i41 = 0o,~i, let i= i+1, and return to step 2.

Then the sequence {Os } is in Oo, it converges weakly to a least favorable a priori distribution

8o E 0o, and ifo, is a minimax estimator 1Moreover the sequence {if } is in to and RP (0, if )

converges uniformly on 8 to RP (0, fo), >

Proof We claim that for any 8 E 8, there exists 0* E 80 such that r"P(0) < rP(0*). Indeed,

fix 0 E 8. Let 8' be such that A(0') = A(0) and r1(0') = -r1(0). It can be verified that since

rl(v) = 0, r"P(0) = rP(0'). Let 0* = ( + '). Then 0* E 0o. By Lemma 7.1, ?"(0) < ?"(0*).

Thus there exists a least favorable a priori distribution 80 that is in 0o. Certainly 8o is also a

least favorable a priori distribution in the estimation problem (0o Pp). Since 0 < p < 1,

RP(0*, 2) = supose RP(0, 2) if and only if R(0*, 2) = supoe, R(0, i). Since 0o is a compact

subset of 8, we may apply Theorem 7.1 for the estimation problem (0o Pp) and derive the

above algorithm. Note that the algorithm is simplified with the help of Lemma 12.1 since we

need to consider only distributions with support of a single point, i.e., elements of 8. Since we

have considered the estimation problem (0o Pp), the sequence {04}@, is in 0o. By Theorem

7.1, the sequence converges weakly to a least favorable a priori distribution 8o E 0o. Thus "0 is

a minimax estimator. Since r1(04) = 0 and rl(v) = 0, if a Lo for all i > 0. Since Condition 7.2

holds, RP (0, ') converges uniformly on 8 to RP(0, Fo).

The main steps of the algorithm of Lemma 12.2 are Steps 2 and 4. In Step 2, we need to

solve the problem of finding a maximizer of R(-, 2) over 0o. We will address this problem in

Section 12.2. Step 4 can be done using numerical methods as described in Remark 7.1.

Suppose V(8, L, R) < Co < Co. Then by Theorem 4.2, there exists a restricted risk Bayes

solution relative to (v, Co). If a minimax estimator for (8, L, RP) and the supremum of its risk

can be calculated for all p e (0, 1), the method discussed in Remark 9.3 can be used to find a

restricted risk Bayes solution relative to (v, Co). By Lemma 12.2, we can assume that a minimax

estimator for (8, L, RP) is in Lo for all p E (0, 1). It can be verified, using (12-3), that if & e to,

there exists 0* E 80 such that R(0*, i) = supose R(0, 2). In general, given an estimator is to~

it may be very difficult to calculate suppose R(0, 2) and the complexity of this calculation may

vary significantly according to 8. Nevertheless, in Section 12.2, we address this problem and are

able to solve it for some important cases of 8.

12.2 Finding a Maximizer of the Risk

In this section, we consider the problem of finding a maximizer of R(-, 2) over 0o, where

is L o, i.e., given~ i Lo, we want to find an element 0* E 80 such that R(0*, 2) =

suppose R(0, 2). This problem is important since we encounter it in step 2 of the algorithm

of Lemma 12.2 and in the method to find a restricted risk Bayes solution, which is discussed

in Remark 9.3. We consider specific cases of the parameter set 0o. There is one immediate

case in which this problem has a simple solution. The definition of an extreme point is needed

(Definition 21). Recall that given a set A, we use dA to denote the extremal boundary of A,

which is the set of all extreme points of A. Since 0o is convex and compact and R(-, 2) is convex

and continuous on 8, by Bauer's minimum principle [41],

sup R(0, i) sup R(0, i).

8680 B6880

If 880 is finite, suppose R(8, i) = maxceaeo R(0, 2). Thus suppose R(0, i) can be easily

calculated.

Fix is L o. By (12-3), R(0, i) = f, tr(As(0t)We) f~or some 91 E SN. 942 6 YN' and

93 E Nzi. Let #4 = {As(0) : 0e 8}0 for i = 1, 2, 3. Let # = {(A(018,n(), A2(0), A3 e E O -

It follows from the definition of 8 that # = GI x #2 x 3. Thus

sup R(0, i)= sup tr I(AsW, I ) sup tr(As~i).

8680 (Ayh,AgA)E6 AgEs

Therefore, we are left with the following optimization problem:

Given NV > 0, a matr~vixr We S"nd a convex compact subse~t A o S,nr

maximize tr(AW) subject to A E A. (12-9)

Consider the important case in which A = { Ae S : ft (A) < ., fr (A) < 0 }, where

fl,. ., frv are convex (real-valued) functions such that A is compact and convex. Then (12-9) is

equivalent to the following convex optimization problem with generalized inequality constraints:

minimize tr(AW)

subject to f()<,i ,.,

A > 0.

Convex optimization problems with generalized inequality constraints can be often solved

numerically as easily as ordinary convex optimization problems [42, pp. 167]. Thus in many

important cases, (12-9) can be solved numerically. In the rest of this section, we consider several

cases in which (12-9) has an analytical solution.

Let Ao a Si" and De R WX" be a nonsingular matrix. Let Wr = (D- ) W~D-l and

el >ea 02 > N denote the eigenvalues of W. Let Be S and At >X~ A2 N be the

singular values of B. Then by a trace inequality of von Neumann [43],

tr(B t)~ <: Agg (12-10)

j= 1

We consider the following possibilities for A:

1) A = {A E S" : tr(DADT) < 1}. Fix AE A. LetX At X~> 2 XN

denote the eigenlvalues of DAD"7. By (12-10), tr(AWrI) =-- trDD 9)

tr AW) < tr DADT Q1. Since A E A, tr A) < Q1. Let A* = D-1UUT D- ), where fi is the

eigenvector of W corresponding to go. Then A* E A and tr(A*W) = gi, whence A* maximizes

tr(AW) over A.

2) A = { Ae S : | |D(A Ao)DTII | |, 1, where |I | || is the Frobenius norm.

Let At >X~ A2 N denote the singular values of D(A Ao)D By (12-10) and

Cauchy-Schw arz inequaity, tr((A Ao) < EAl {g.Tu

tr(AW) < ||D(A Ao)DT||y||W||y + tr(AoW). Since A E A, tr(AW) < ||9||,l + tr(Ao ).

Suppose A* E A and tr(A*W) = ||9|| + tr(AoW). Then

tr (A*n,~ Ao)D = 1.

Since the assignment (A|B) = tr(AB) yields an inner product on the space of real-valued

NV-by-NV matrices and ||D(A* Ao)DT||, < 1, we have by the Cauchy-Schwartz inequality

that D)(A* Ao)D7 = ~iT'hus A* maximizes tr(AWI) over A if and only if A* =

Alo + D1'I(D 1)T

3) A = {A E S" : ||D(A Ao)DT||2 < 1}. Fix A E A. Let A1>X~ A2 N

denote the singular values of D(A Ao)DT. By (12-10), tr((A Ao) ') < X1 CE= gy. Thus

tr(AW) < tr(W)||ID(A Ao)DT||2 + tr(AoW). Since A E A, tr(AW) < tr(W) + tr(AoW). Let

A* = Ao + D-I(D-1)T. Then A* E A and tr(A*W) = tr(W) + tr(AoW), whence A* maximizes

tr(AW) over A.

4) A = {eAo + (1 e)A : ||DADT||y < 1}, where 0 < e < 1. Certainly A*=

eilo + (1 e) D-1 1)T maximizes tr(AW) over A(. We canl replace | | || in the definition of A

by tr(-) or || ||2~ and have analogous results.

Let us consider an example in which we find a maximizer of tr(At Wi) over Gr.

Example 12.1. Suppose the set 31o is compact in the sense of the usual Euclidean norm and

symmetric around the point 0, i.e, xo E 310 if and only if -xo E 31o. Let MZ/xo denote the set of all

probability measures on (31o, a(31o)). Suppose it is known that the true distribution of the initial

state belongs to MZ/xo. While MZ/xo has elements that are not Gaussian, for each such element

there corresponds a Gaussian distribution with the same mean vector and autocorrelation matrix.

Since the risk functions depends only on the mean vector and autocorrelation matrix, the results

of thi\ chanpter can be used~. LePt 81 = {((9, A) : 17 = fxoxody, A = fxoxrox~r, reMxo

Since 31o is compact, MZ/xo is weakly compact. Let {-ri}@, be a sequence in MZ/xo that converges

weakly to -ro E MZ/xo. Then clearly {((qi, As) }g, converges to (rlo, Ao), where rli and As are the

mean vector and autocorrelation matrix of -ri (i = 0, 1, ..), respectively. Since MZ/xo is compact,

81 is compact. Since MZ/xo is convex in the usual sense, 81 is convex. Fix (rl, A) E 81. Let

r E MZ/xo be such that rl and A are the mean vector and autocorrelation matrix of -r, respectively.

Let -' be defined as follows: for every Ce B (3o), -r'(C) = -r(-C), where -C = {-xo : xo E C)

and is in B(31o). Ct, icirrh -' E MZ/xo, r' = -rl, and A' = A, where rl' and A' denote the

mean vector and autocorrelation matrix of 7', respectively. Thus (-rl, A) E 81. Recall that

GI = {At (0) : 0 E 8 }. We want to find a maximizer oftr(ARl) over Gr.

Suppose 3o = {xo E RWN : x"DTDxo < 1}, where D E RWN~xN is nonsingular

Fix Ae t r. Then there exi'Fs ~tse Mxo such? that AZ = fXoxoxz~cdr. Th~us tr(DADT')=

fxo tr(Dxox"D..TT)d~r < 1. Let Ae S N.1 and suppose tr(DADT) < 1 and A f 0. Then

C As < 1, where As is the eigenvalue of DADT corresponding to the ith eigenvector ui.

Certainly DADT = C Asn.LT Thus A = C Asibil', where As = Ag/(C Ay) and fig =

CjDI14 for i = 1, N. Since, ifDTZ)i = C As < 1, ~ig E Xo and -~ig E Xo

for i = 1, .. ., Nz., Let -r* be the distribution that assigns mass Ag/2 to fig and Ag/2 to -fig

(i=1, I. N). Thenr r* E MxoLs, has zero mean, and A = x xox~d~rdr*. Thus Ae E 1 It follows

that At = {A E SN. : tr(DADT) < 1}. This case was alreadyv treated in rlhi\ section.

Suppose 31o is a convex polytope that is symmetric around the point 0. Then there ex-

ist a finite number of points Xo,1, X0,2, 0 X,N E 30 Such that xo,i and -xo,i are extreme

pointsf of Zofor i: = 1, 2, ..., N. Certain~ly sup~,,l tr(A~l) = suprerxo So tr(xrox Wi~)dv <

suzeotr/xo -W'1). By Bauer's minimum principle [41], supzoexo tr/xo "W'1) = supzoeaxo tr(xox ~).

Since 31o has a finite number of extreme points, sup,,,, tr (A"

Since xoi,~ix E 01 for i = 1, ..., NV, suphE4 tr(A~l) = Inaxli~aN tr(xo,ix,~i 'Y) andU therelt

exists 1 < i < NV such that xo~ix", maximizes tr (Al) over Gr.

12.3 Connection to the Kalman Filter and E-Minimax Approach

Let igln denote an estimator for xm based on the observations II,,. Let F(0, iml,) denote the

MSE matrix of the estimator iml, when 8 is the true state of nature. Let 2,(0) and in,_-1(0) be

defined by the well known update equations of the KF:

Bo -1(e) = r11(0), (12-11)

Fo-()= A,~,(0), (12-12)

K,~(0) = Ps,_n1(0)HI[Ay,2( + H,F4,l_l(0)H ]- (12-13)

8,(0)=e>10 ,0)[ -Hi,10] (12-14)

r,(0) = [I K,(0)H,]F, (0)(8, (12-15)

in1()= F,2,(0), (12-16)

I', ,l(0) = Ay,~3(H) + Elnful0)Fu (12-17)

where 0,(0) = F(0, 2,(0)), r,,,_,(0) = r(0, l4,,_,(0)), and K,(0) denotes the Kalman gain.

Consider the filtering problem specified by the estimation space (S,, S,, h), where S,=

{0,. ., 71<}, S, = N, and h(i) = {0,. ., i} for each i E S,. Then is = (o (8), 11(8), in, (8>>

and is referred to as the KF relative to 0. Since A2(0) > 0, the existence of the KF is guaranteed

[44]. It is easy to verify that Rk (8, k (8 )) = trW1~B k8, k8 ))).

Using the results of the previous sections, there exists a 0 E 8 such that the KF relative to

8 is a restricted risk Bayes solution relative to (v, Co) for all Co > V(8, L, R). In fact, there is

another interesting property regarding the KF. Using Corollary 9.1 and Lemma 12.1i, we have

that the class of KFs relative to H E 8 is essentially complete. Thus as long as the performance

is judged solely based on the risk function, if the choice of estimators is restricted to affine

estimators, then no matter what optimality criterion is used, one may consider only the class of

KFs relative to H E 8.

Our next step is to derive more convenient expression for Rk(8, k~(0')) in the case that 8 and

8' are in 0o. Let Uk 8, 8/ k (,~ (0')). Using (12-13)-(12-17), it can be shown that

Fo 1(, ')= A (0)

', (0, H') = K, (0') A2 0 K, (0') + [I K, (0') H,] 4, (0, H') [I K, (0')H,]T

Since R, (8, k ,8 )) = tr( k k(0, 8')), we have a more efficient way to calculate the risk

than through (12-3). Recall that if H and H' are in 0o, then R(0, fe,) = C,_ tr(A4i(0) We(0)).

Our goal is to find an expression for We(0'), for i = 1, 2, 3, since it is needed in order to find a

maximizer of R(-, is,) over 0o

Let F,(8/) = F,-Fx,t(BI)H,, 1et Fi,j(0') = i(0')Ni_,(0') Fj(0') (i > j) and Fi,i(0') =

Fi(0'). We will use the convention E-_l1,4(0') =ILe rm0)=(I-Kt()r)-,m)

(0 < m < n, O < n), let Dn,m(8/) = Gr,m+1(8/)F, >Ka(0) (n > m > 0) and D,~,(0') = K,(8/)

(n > 0). Note that C+1~,m = (I K,+1H,t+1)Exs,m and Cr,-1 = Gr,mF,-1. Then

Gro(0')Az(0)Gro(0') + Dnm(0')A2(0)Dnm(0') + Grm(0')A3 8 rtm 8

m= o m= 1

r,(e, e')

Let Ti(0') = CE" >= ,~i(BI) 914zGi(0') for i = 0, 1, .., us. Then

It follows that

n=0

n=0 m=0

m=1 m=0

n=l m =l m =

In general, the calculation of W2(0') and 93 0 ) TCCJUifeS the storage of Ko(0'), .., K,t (8'),

which may be problematic for large us. This is due to the fact that Ti can be updated based on

Ti+l but not vice versa. Nevertheless, in the important case that R, is invertible for 0 < i <

us 1, the calculation of Wi, 92, and 93 may be done in such a way that there is only a need of

a fixed storage place that does not depend on us. First, let us show that since A2 0 ) iS invertible,

I K,(0')H, is invertible. By (12-13),

I K(0')H = I 0,1(0')H [nA28) HnF4,_1(0)H ]- Hn

Thus by the matrix inversion lemma, I K(,(0')H1, is invertible and

(I K,(0')H,)-1 = I + 04,_-1(0')If"A2 pl) -1H,.

Since Fi is invertible, T441 = [(I K Hi)- F- ] T4I KeH ]- F-' (F- ) H'sF- In this

case, Ti41 can be updated based on Ti and only a fixed storage place is needed.

We now discuss the connection between the restricted risk Bayes approach and the 0-

minimax approach and illustrate that the F-minimax approach can be regarded as a special case

of the restricted risk Bayes approach. The class 0 of a priori distributions in the F-minimax

approach coincides with the class 8 of the states of nature in the restricted risk Bayes approach.

By setting Qo = V(8, L, R), we have that for any -r Me Zl, a restricted risk a Bayes solution

relative to (-r, Qo) is a minimax estimator for (8, L, R). Since 8 = 0, the risk R(0, x) is, in fact,

the Bayes risk relative to a certain distribution in 0 if we adopt the F-minimax formulation. Thus

a minimax estimator for (8, L, R) is a F-minimax estimator. Therefore, the results of this work

can be used to find a F-minimax estimator. We note that if some a priori information is available,

the restricted risk Bayes approach is preferable to the F-minimax approach since it utilizes this

information. However, if no a priori information is available, the most reasonable choice for C6

seems to be V(8, L, R) and hence we are left with F-minimax estimation.

12.4 Numerical Example

To illustrate the theory of the previous sections, we consider the following simplified

problem as an example. Suppose a target is moving in the one-dimensional space. We assume

the three-state track model [45]. Let x, E RW3, where x,(1) denotes the position of the target,

x,(2) denotes the velocity of the target, and x,(3) denotes the acceleration of the target. We

also assume a radar measures the position of the target. Hence the state space model in (11-1) is

specified with

1 a A2/2

Fu = 0 1 a

00 1

Ir,, is a zero-mean random vector with nominal covariance matrix

000

G~o = 0 0 0

H, = [1 0 0], and v, is a zero-mean random variable with nominal variance Xo. We assume

xo E 31o, where 31o = {xo E RW3 : x DTDlxo < 1} and that a nominal distribution for xo

is available. The nominal distributions of xo, In~,,. and v, are available from past experience but

are not assumed to be the exact distributions. The exact distribution of xo belongs to MZ/xo, the

space of distributions defined on (31o, a(31o)). The exact distribution of the measurement noise is

known to belong to the class of zero-mean distributions whose variance A satisfies | A Xo I I x

for some rx > 0. Similarly, the exact distribution of the model noise is known to belong to a class

of zero-mean distributions whose covariance Q satisfies ||ID(Q Qo)DT||y < r, for some rg >

0and De R N"XN. By Example 12.1i, Pi = {AE : tr(D1ADT ) <' 1}. In addition, it s ler

that #2~ = {AES : ~ ||D2( 0) || < "1} and #.3 = {Q ES :- ||D3 0D|y }

where D2 = --1/2 and D31 r--1/2D. In this example, we consider the filtering problem specified

by the estimation space (S,, S,, h), where S, = {0,..., nt}, S, = N, and h(i) = {0,..., i} for

each i E S,. We choose the following values: a = 0.01, at = 120, Ao = 1 x 104, g0 = 100,

rx = 1000, r, = 30,

5 x10-6 0 0

DI = 0 0.002 0

0 0 0.01

2/A2 0 0

0 0 1

We assume that the nominal distribution of xo has zero mean and diagonal covariance with

diagonal entries (1 x 106, 1000, 10). We consider the risk based on quadratic loss functions

as our performance measure and let the weight matrix W,, for each 0 < n < us, have all zero

elements, except only for the unity element in its upper left hand corner. This choice of the

weight matrix implies that we are only interested in estimating the position of the target and we

regard the velocity and acceleration of the target as nuisance parameters.

Clearly Assumption 12.1 holds in this example. We have shown that Assumption 12.2 holds

in Example 12.1. It can be easily verified that Assumptions 12.3 and 12.4 hold as well. Our

first step is to choose Co. In order to choose Co, we need to calculate V(8, L, R) and Co. Once

these quantities are available, we can choose Co based on the amount of a priori information

we have. Let &;r denote the (essentially unique) minimax estimator in the estimation problem

(8, L, RP). We can get an insight as to how to choose Co by plotting the Bayes risk r (v, & )

versus the maximum risk supoe, R(0, & ). We use the algorithm of Lemma 12.2 to calculate

A* for per [0, 1) and then calculate r(v, & ) and supose R(0,~* & .Fig 12-1 showsr the plot of

the Bayes risk r(v, &* ) versus the maxIimumlll risk~ suose p U~aCI ,u,,,,,IU~vr R(0, &* ) Note that byI Theorem1 9.1,

this figure, in fact, shows the Bayes risk achieved by a restricted risk Bayes solution relative to

(v, Co) versus Co. Hence this figure tells us the tradeoff between the penalty on the Bayes risk

and the safeguard on the maximum risk by employing restricted risk Bayes estimation. In this

example, V = 1.225 x 10s and Co = 6.474 x 106. We can see from the figure that on one

hand, it would make little sense to choose Co > 8 x 10s since the improvement in the Bayes risk

would be minor and the degradation in terms of the maximum risk would be very significant. On

the other hand, if we choose Co to be very small, we would have only a minor improvement in

terms of the maximum risk and a significant degradation in terms of the Bayes risk. Therefore,

it seems that in most cases, except maybe the case of complete lack of a priori information, we

would choose 1.5 x 105 < Co < 8 x 105. Again, the exact choice of Co depends on the amount

of a priori information we have and therefore is done heuristically. For illustration, we choose

Co = 5 x 105. We calculate the restricted risk Bayes solution relative to (v, Co) using the method

discussed in Remark 9.3 and the algorithm of Lemma 12.2. It turns out that the restricted risk

Bayes solution relative to (v, Co) is a minimax estimator for the estimation problem (8, L, RP"),

where po = 0.986. With this choice of Co, the restricted risk Bayes approach can reduce the

maximum risk to about 1/13th of that of a Bayes solution while suffering only minimally (about

4%) on the Bayes risk.

Fig. 12-1 illustrates the behavior of the Bayes, minimax, and restricted risk Bayes solutions

in two extreme cases: The case that v is the true state of nature and the case that the true state of

nature is the worst case choice for each one of these estimators, respectively. However, Fig. 12-1

does not illustrate the behavior of these estimators for other values of the true state of nature.

It is important to evaluate the performance of these estimators relative to all 8 E 8 that are

likely to be the true state of nature based on our a priori information. In order to do that, we

need to make some assumptions regarding the set of 8 E 8 that are likely to be the true state

of nature. We assume that this set is 8, = {eov + (1 co)8 : e < co < 1, 8 E 8},

where 0 < e < 1, i.e., the true state of nature is likely to be an co-mixture between v and

an unknown 8 E 8 for e < co < 1. The more a priori information we have, the more

likely it is that the true state of nature is close to v and hence the larger e is. Note that for any

:i E L, supose, R(0, i-) = er(v, i-) + (1 e) supose R(0, i-). Let i-* denote the (essentially

unique) minimax estimator and recall that :i-, denotes the (essentially unique) restricted risk

Bayes solution relative to (v, Co). It is easy to verify that supose R(, 'e) = e?"(v) + (1 e)Co,

supeoe, R (0, i*) = er (v, -*) + (1-e) V(8, L, R), and suppose R(, )~ = er (vI, i*) + (1- e) Co

Let us assume that there is a sufficient amount of a priori information so that e > 0.95. Fig. 12-2

shows the maximum risk over 8, of the Bayes, minimax, and restricted risk Bayes solutions for

e E [0.95, 1]. This figure shows the worst case performance of these estimators relative to values

in 8 that are likely to be the true state of nature. The maximum risk over 8, of the restricted

risk Bayes solution is less than that of the Bayes solution for almost the entire range of e. In

the interval [0.95, 0.98], the maximum risk over 8, of the restricted risk Bayes solution is in

fact significantly less than that of the Bayes solution. The maximum risk over 8, of the Bayes

solution is less than that of the restricted risk Bayes solution only for e > 0.9995 and even in

this case, the difference is very small. It can also be seen that even a relatively small uncertainty

in the true state of nature may lead to undesirable performance of a Bayes solution relative to v.

For example, in the case e = 0.98, using the restricted risk Bayes solution instead of the Bayes

solution leads to 50I' improvement in terms of the maximum risk over 8,. In addition, it can be

seen that the performance of the restricted risk Bayes solution is superior to that of the minimax

estimator for all e > 0.95. This illustrates that if a priori information is available, the restricted

risk Bayes approach is preferable to the somewhat conservative minimax approach.

105x104

o Minimax Solution

03

95-

06

9-"

85-

S09

75- FO~

0 975 Unrestricted

Bayes Solution

0 999

650135

Ssupp R(9, f) x lo'

Figure 12-1. Achieved Bayes risk vs. maximum risk

x 105

47

Bayes Solution

Minimax Solution

Restricted Risk Bayes Solution

3 5

3-

25-

15-

05

095

0 955 096 0 965 097 0 975 098 0 985 099 0 995

Figure 12-2. The maximum risk over 8, of the Bayes, minimax, and restricted risk Bayes

solutions vs. e

CHAPTER 13

STATE ESTIMATION WITH INITIAL STATE UNCERTAINTY

In this chapter we consider another special case of the state estimation problem of Chapter

11. We consider the case of deterministic uncertainties in the initial state. We assume that the

sequences {v,} and {tt',,}), which have zero mean, satisfy for n, m > 0

G~v~v0, otherwise

0 otherwise

E(vow )= 0, (13-1)

where E denotes the expectation operator. This means that 82 and 83 COntain only one element.

We restrict ourselves to the case A2 > 0, which is usually the case in well modeled problems

[44]. It is left to specify the initial state vector xo. We model xo as a deterministic (unknown)

parameter belonging to a parameter set No0. We assume 31o is a compact subset of RWN. Note

that the current formulation deviates from the standard KF assumptions in that the initial state is

not modeled as random with known statistics. Since we assume deterministic uncertainty in the

initial state vector, 81 is the collection {(xo, xox ) : xo E o}0 of mean and second moment

pairs. Note that since 31o is compact, 8 is compact.

We are interested the filtering problem that is specified by the estimation space (S,, S,, h),

where S, = { 0,. ., at }, S, = N, and h(i) = { 0,. ., i } for each i E S,. We let w (i) = 1 for

each i E S,, i.e., the risk function is simply given by

n= o

Since there is a one-to-one correspondence between 8 and 31o, we slightly abuse the notation and

assume that the space of states of nature is 31o, i.e., in this chapter 8 = Zo0. We also use R(xo, 2)

instead of R(0, 2), R,(xo, 2) instead of R,(0, 2), for n = 0, ..., us, and etc.

13.1 Conditional Mean Estimators

Recall that rI,, = s yTU T]". Given -r Me Zl, let 2,(-r) denote the mapping

y"n H Exo(X,|y"n), i.e., 2,(-r) is the conditional mean (CM) estimator with respect to -r when

xo is the true state of nature. Let F,(-r) denote the MSE matrix of 2,(-r) with respect to -r. Note

that we may regard a point xo E 31o as an element of M /xo by regarding it as the probability

measure that assigns mass 1 to xo. Thus 2,(xo) and F,(xo) are well defined. Let 2(-r) =

(f-o(T),., i In,(7)j). Recall that r(-r, ) = fxo R(xro, 2)dr. In addition, we use the following

simplified notation: r(-r) = r(-r, 2(-r)). It is easy to verify that r(-r) = CE,o tr(W,F,(-r)), where

We = Vs"v,.

Let o dennote the conditional mean estimator whel~n xo = and let I`o denote the MSE

matrix of fo when xo = 0. It is clear that io and yo can be carllclated usingT the well kmnown KF~

recursions [44] by initializing the KF with zero mean and covariance. It is straightforward to

verify that F,(XO) = 1o for all o r Zo. Lt n 3+ POn FI K = l- n TA

H,o,_ znlHI] -1for n > 0,and ~o= 0. Let F,= F,- F,,KoH,,1 et FL~3 = FF_1--- Fy (i > j)

and Fi~i = Fi. We will use the convention Fi_,i = I.

The problem of state estimation with a random initial state whose distribution in not

necessarily Gaussian is considered by Lainiotis et al [46] and the CM estimator for this problem

is derived. Let io,n(-r) denote the CM estimator with respect to -r for xo based on ti,,, i.e.,

io Sxo") =~~)~ n e o.()=foEx (onr -x)i~) -x)) dr. Using

the results of [46], it is not difficult to get the following set of equations for n > 0:

2,(r) = ain ~ ) (13-2)

C,=( o,),1,1=(I KoH,)F,-1,o, a > 0, (13-3)

Co = I, (13-4)

F()= Fo + CnCo,n('r)C,, (13-5)

U, H AgHo+ i 1,~-,oH~, (Her _llZH, + A2 -1 He e1,o, (13-6)

i= 1

tn (Y") = Ho Ag2 Yo + i 1,oH, (Her _zH,7 + A2 -1 s A _z (13-7)

i= 1

p" (y") oc exp [T U,o- 2xT ty" gy) (13-8)

where the term go is a fu~nction of y1 and its exact calcu~latin on isnnecessarry for our purpose

Thus 2,(-r), the CM estimator with respect to -r for x,, is the sum of two terms; one term is

io, the CM estimator for x, when the system has zero initial conditions, and the second term

is completely specified by io,n('r), the CM estimator with respect to -r for xo. An alternative

interpretation is to regard 2,(-r) as the sum of two terms, where the first term is an estimator of

x, for known zero initial condition and the second term accounts for the effect of the unknown

initial state. A conditional mean estimator with respect to -r is an essentially unique Bayes

solution relative to -r. The expression for the conditional mean estimator is important since an

essentially unique minimax estimator is the conditional mean estimator with respect to a least

favorable a priori distribution. In addition, the class of conditional mean estimators with respect

to -r M i/xo is essentially complete. Let ~D = { (7) : re xo iZ1X)

13.2 Approximations to Minimax Estimators

In order to find the (essentially unique) minimax estimator, we need to derive a conditional

mean estimator relative to a least favorable a priori distribution. Hence we are left with the

following dual problem: Find 7o E Mxo, such that r(7o) =sup,eMso r(7). This dual problem

is conceptually easier than finding a minimax estimator directly by the definition. Nevertheless,

solving the dual problem may still be a difficult task. Thus it may be necessary to search for

sub-optimal estimators in terms of the maximum risk as suggested in Chapter 8. Our goal is then

to derive suboptimal estimators that can give maximum risk arbitrarily close to that of a minimax

estimator. The following Lemmas are derived in [24].

Lemma 13.1. Suppose 31o is compact. Then if {8 } is a sequence in MZ/xo that converges weakly

to To E MZ/xo, then R(xo, Sh))> converges to R(xo, 2(To)) uniformly on No0.

Lemma 13.2. Let f e D Then there exists an extension of R(-, 2) from No0 to an open convex

set 31o C Uc RWN such that R(-, 2) is twice difgerentiable on U. 1Moreover for all xo E 310

D2 2 0o) < 2U,, sup R(xo, 2). (13-9)

zo6Xo

Because of Lemmas 13.1 and 13.2 and the fact that 31o is a subset of finite dimensional

normed space, we can use the results of Chapter 8 to find a y-optimal estimator. In particular if

31o is a convex polytope, Lemma 8.7 can be used to find a y-optimal estimator. In order to find

a y-optimal estimator, we need to set a 0 < y' < y and construct a sufficiently dense finite

subset 31o' of 31o according to the lemma. We then need to find a (y', 3lo')-optimal estimator.

The lemma tells us that this estimator is a y-optimal estimator. A (y', 31o')-optimal estimator

can be found using, for example, the algorithm that is proposed in [38] and that also appears

in [18] and Theorem 7.1. Using Theorem 7.1, it is not difficult to verify that all the necessary

conditions for this algorithm to be valid hold. It worths mentioning a technical difficulty in this

approach. For us to use the algorithm of Lemma 7.1, we need to calculate matxroeXof R(xo, NT)),

which means that we have to calculate the risk R(xo,2(-r)) for each xo E 31ol. However, an

analytical expression for R(xo, (7)), for xo t Xof and 7 e Mzoy, is not available, in general.

Hence R(xo,2(-r)) must be calculated numerically for all xo E 31o'. Thus the resulting numerical

algorithm involves intensive multi-dimensional integration at each step. Due to this difficulty, it

is desirable to use efficient numerical integration methods. In [24] there is a discussion regarding

how the numerical integration can be done efficiently. Also, in [24] an alternative algorithm to

the one of Theorem 7.1 is proposed. It is argued that this algorithm may be preferable to the one

of Theorem 7.1, in terms of the computation burden, since it may require less calculations of the

risk function.

The computational complexity of a y-optimal estimator derived according to Lemma 8.7

depends linearly on the support of -ro. It may be rather high, and it is certainly higher than the

complexity of affine estimators. In Chapter 12 we discussed the problem of restricted risk Bayes

estimation when the class of estimators is restricted to affine estimators. When the class of

estimators is restricted to affine estimators, the minimax problem of this section can be regarded

as a special case of the problem considered in Chapter 12. If we can find an affine estimator that

is also a y-optimal estimator, we would probably prefer the affine estimator. If such an affine

estimator exists, the results of this section are still important since they give us tight lower and

upper bounds on the maximum risk and hence enable us to evaluate the performance of the

affine estimator with the best possible performance in the sense of the maximum risk. We further

illustrate this in the following numerical example.

13.3 Numerical Example

We consider the following problem as an example. Suppose a target is moving in the one-

dimensional space. We assume the following simplified model of motion. Let .r,z E R2, where

.r, (1) denotes the position of the target and .r, (2) denotes the velocity of the target. We assume

r,w+1 = Fr,z + 1,',,, (13-10)

where

and I,',, is zero mean Gaussian random vector with covariance matrix

00 ,

We assume xo E 31o, where 31o is a closed rectangle in RW2. We also assume a radar measures the

position of the target and the observations from the radar obey the following equation

yn = Hx, + v,, (13-11)

where H = [1 0] and v, is a zero-mean Gaussian random variable with variance A2. NOte that

this example is a simplified version of many real applications in which the KF is used [21].

In this example, we assume that 31o = [-1000, 1000] x [-50, 50], a = 0.01, A2 = 2500,

q = 25, at = 10, W, = 0 for n < 5, and W, =, for n = 5,. ., us, where W, = V,TV,.

Our goal is to derive a y-optimal estimator for y = 0.03. This means that the degradation,

when using the derived estimator instead of an exact minimax estimator, is at most 3%b of the

maximum risk. The admissible y-optimal estimator that we derive is a CM estimator with respect

to a discrete a priori distribution.

Let V be a matrix such that the ith column of V is the ith eigenvector of 2Uto. Let 6i denote

the ith eigenvalue of 2Uto. Let y = 0.03 and y' = 0.0023. The above choices of y' results from a

certain tradeoff. The choice of y' determines how dense the finite subset 31o' of 31o and specifies

h~ow close the resulted estimator is to a minimax estimator for (No', D,;: R). If we choose y' very

close to zero, we would decrease |Xo| but we woumld need more iteratinon to find a (y', Ro)-

If we choose y' too large, the dimension of 31o' would grow and hence also the computational

complexity. Let b6() = f--i- Ior i = 1, 2. We construct a finite set Xo' such that Xo' is

(6, V)-dense subset of 31o, o', n 31,b is (6, V)-dense subset of 31,b, where 31,b denotes the boundary

of Ro, and the points (-50, 1000), (50,! 1000), (-50, -1000), and (50, 1000) belong to No'f. The

set 31o' is plotted in Figs. 13-1 and 13-2. Due to the eigen-structure of the matrix Uto, the resulted

set 31o' is not a standard grid. The points in the set 3lof are much denser in the direction of the first

eigenvector than in the direction of the second eigenvector.

Next, we solve for an (y', 31o')-optimal estimator. We set the initial a priori distribution to

be the uniform distribution on 31o'. We then update the a priori distribution using a variant of the

projected gradient method as discussed in [24]. As mentioned earlier, it is possible to use also the

algorithm of Theorem 7.1. Each update increases the Bayes risk. We stop the algorithm when the

required tolerance of y' is achieved. Since a (y', Ro')~-optimal estimator is an e-minimax estimator

for (N ,~ 1D, R) with~ c = V/(o D, R), it is possible to check< whether th~e required tolerance is

achieved using Lemma 8.1i. Let -ro denote the resulted a priori distribution; the resulted estimator

is then 2(-ro), which is a y-optimal estimator. Fig. 13-3 shows the risk R(xo, 2(-o)) for xo E 31o .

The maximum risk on the set 3lof is 1864.6. Due to the construction of 31o', the maximum risk on

31o is less than 1916.1i. In Fig. 13-4 we show the resulted a priori distribution Tro.

It seems unreasonable to compare between the proposed y-optimal estimator with an

estimator that is derived using the F-minimax approach. The reason is that by assuming that

xo is deterministic and belongs to 31o, there is no systematic way as to how to choose a class 0

of distributions. Of course, one can choose E to be MZ/xo, but then a F-minimax estimator can

be shown to be a minimax estimator (i.e., the two problems are equivalent). It may be the case

that a properly initialized KF can have a very close performance to that of a minimax estimator.

If one adopts the formulation of this work, then a rigorous way to initialize the KF is to derive

a linear (or affine) minimax estimator, which is a KF with respect to a least favorable a priori

distribution. In practice, the KF is often initialized with zero mean and covariance a2I, where

e2 iS chosen heuristically. We wish to illustrate that this heuristic method can sometimes lead to

undesirable performance. In Fig. 13-5, the maximum risk of the KF initialized with zero mean

and covariance a21 iS plotted as a function of a e [150, 450]. It is apparent that choosing a

too small leads to a very poor performance. In addition, choosing a too large may also lead to

undesirable performance. The best choice of a in terms of the maximum risk is a = 289.7 and

the maximum risk in this case is 2042.8. Thus for the best choice of a, we have a degradation of

at least 6.5% and at most 10%b in terms of the maximum risk relative to the estimator 2(-ro). This

is not a significant degradation and when complexity is taken into account, it may be preferable

to use a KF that is initialized with a = 289.7. In this example, it is clear that the degradation if

a linear minimax estimator is used instead of a minimax estimator is at most 10%b and may be in

fact much less. This should not imply that the whole construction of i(Tro) is unnecessary. It is

this construction and the calculation of maxsoexo; R(xo, 2(7o)) that enabled us to establish that a

linear minimax estimator has a very good performance in this case.

S20-

10 -0

-0-

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

xo (1) (initial position [m])

Figure 13-1. A full view of Ro',f which is a finite (6, V/)-dense subset of Zo

-46-

S-47-

~-47 5-

S-48-

;-48 5-

-49-

-49 5-

-50- *

-1000 -950 -900 -850

xo (1) (initial position [m])

Figure 13-2. A zoom-in view of the bottom left corner of No', which is a finite (6, V/)-dense

subset of 31o

1900 x

1800-

1700 m

HC1600 m

1500 m

1400

50

1000

0 500

-500

xo(2) (initial velocity [m/s]) -so -1000 xo(1) (initial position [m])

Figure 13-3. The risk of x(ro), the derived y-optimal estimator, as a function of xo E o,~

x 10

1000

-500

xo(2) (initial velocity [m/s]) -so -1000 xo(1) (initial position [m])

Figure 13-4. The a priori distribution -ro, which is defined on 31o

2900-

2800-

2700-

6 2600-

S2500-

S2300-

2200-

2100-

2000-

1900-

150 200 250 300 350 400 450

The maximum risk of the Kalman Filter initialized with zero mean and covariance

O.2I as a function of o-

Figure 13-5.

CHAPTER 14

CONCLUSIONS

We considered the problem of state estimation with stochastic and deterministic uncer-

tainties in the initial state, model noise, and measurement noise using a decision theoretic point

of view. We considered both the case that the class of available estimators is the class of all

estimators and the case that the class of available estimators is the class of affine estimators. We

showed that a minimax estimator and a restricted risk Bayes solution exist when the risk is based

on quadratic loss functions. Under further conditions, a minimax estimator can be found as a con-

ditional mean estimator or a LMMSE estimator relative to a least favorable a priori distribution.

In the case that all the uncertainties are stochastic, we adopted the restricted risk Bayes approach,

which incorporates the use of a priori information to derive estimators that are robust to devia-

tions of the model from the nominal assumed model. We derived a general method to obtain a

restricted risk Bayes solution. When the class of estimators is restricted to affine estimators, this

method can be easily used provided that a maximizer of the risk function can be calculated. We

considered several important cases in which a maximizer of the risk function could be calculated

analytically and showed that in many other cases it could be calculated numerically. Thus we

provided a systematic way to derive restricted risk Bayes solutions. When the filtering problem

is considered, the restricted risk Bayes approach provides us with a robust method to calibrate

the Kalman filter. We also considered the problem of state estimation with deterministic initial

state uncertainty. In this case, we proposed a numerical method to derive an approximation for

a minimax estimator with the possibility to make the approximation as accurate as desired. This

method seems to be especially attractive in the case that the parameter set is a convex polytope in

APPENDIX A

PROOF OF LEMMA 5.2

Let w' be the system of subsets B C w* such that oo a B and C* \ B is a closed, compact

subset of C in the sense of the topology w. The topology w* is such that each set in w* is a union

of a set from w and a set of w'. It is well known that w is, in fact, the relative topology on C. Since

C is locally compact, o--compact, and metrizable, C* is compact and metrizable.

Let us show that the f(a, -) is lower semicontinuous on C* for each a s A. Fix a point

c* E C. Let {c,} be a sequence that converges to c* in the sense of w*. Then {c,} is eventually

in any w*-neighborhood of c*. Since (C*, 0*) is Hausdorff, there exist disjoint open sets A*

and B* such that A* is a neighborhood of c* and B* is a neighborhood of 00. Since B* is

open, it is the union of an element of w' and an element w, and we can assume without loss

of generality that it is in w'. Let A be an open w-neighborhood of c*. Then A U A* is an w-

neighborhood of c*. Since {c,} is eventually in A U B* and eventually in A*, {c,} is eventually

in A. Thus {c,} converges to c* in the sense of the topology w. By the hypothesis of the lemma,

lim inf,,, f (a, c,) > f(a, c*). It follows that lim inf,,, f (a, c,) > f(a, c*). Let {c,} be

a sequence in C that converges to oo. Then for each A E w', {c,} is eventually in A. Thus for

each w-compact subset C' of C, there exists NV > 0 such that c, ( C' for n > NV. Let {C,} be

a sequence of compact subsets of C such that U" zC, and c, Sf C, for (n = 1, 2, ..). Since

lim inf,,, f (a, c,) = supec f (a, c), lim inf,,, f (a, c,) = f (a, 00). It follows that f (a, -)

is lower semicontinuous on C* for each a s A. Clearly f is nonnegative. Since w is the relative

topology, each Co C C that is in B(C*) is in B(C). Let Co a w. Then since Co U co E w* and

C* \0 oo E w*, Co aB(C*). If Co aB(C), it is a countable union and intersection of elements of w;

hence a countable union and intersection of elements of B(C*). Thus Co aB(C*). It follows that

if Co c C, Co aB(C) if and only if Co E a(C*).

Suppose f(-, c) E m(FA~) for each c e C. Then clearly f(-, c) E m(FA~) for each c e C.

Since f (-,oo) = supeec f (-, c), f (-, 0) E m(FA~). Thus f (-, c) E m(FA~) for each c e C*.

Suppose fe E (FA x B(C)). Let A be a Borel set of the extend reals. The set f-1(A) =

{(a, c) E Ax C* : f (a, c) E A}. Then f -1(A) = {(a, c) E Ax C : f (a, c) E A} U {(a, 00):

a s A, f (a, 00) E A}. Clearly f -1(A) = {(a, c) E Ax C : f (a, c) E A} is in FA~ x B(C). Hence

it is in FA~ x B(C*). Since f (-, 0) E m(FA~), the set f -l(-,oo)(A) = {a EA : f (a, 00) E A}

is in FA~. Thus {(a, 00) : a s A, f (a, 00) E A} = f -l(-,oo)(A) x {oo}, and hence is in

FA~ x B(C*). It follows that f"-1(A) E Fg~ x B(C*). Thus fe m(FA x B(C*)).

APPENDIX B

PROOF OF LEMMA 9. 1

Note that for any~i 6 E O < pi, p2 < 1, and 0 < rl < 1,

The concavity of K follows from the fact that for any 0 < rl < 1,

K~vyt +(1 77p2)= inf .K(:i, rlpt + (1 rl)p2)

= rlK(pl) + (1 rl)K(p2 m

where the inequality above is a direct result of (B-1). Eqn. (9-1) implies that K(pl) > K(p2)

for any 0 < pi < p2 < 1. Since K is decreasing and concave on [0, 1], it is continuous except

perhaps at the point p = 1. Certainly K(:i,., 1) = K(1) and K(:i,., p) > K(p) for all 0 < p < 1.

Thus for any 0 < p < 1,

| K(p () |< (f e, ) -K :fe.,1)= ( -p)sup R (0, :f e) r (v, :f e.)]< (1 p) Co

where the last inequality results from our assumption that R is nonnegative. This proves that K is

continuous at p = 1 and thus on [0, 1].

APPENDIX C

PROOF OF LEMMA 9.2

1) It is easy to see that

K(2 2) = K(p2) < K(ix, p2) < K(ix, pt) = K(pt) < K(2 1

(C-l)

where the second inequality is a direct consequence of (9-1).

This implies that K(2, 1) K(2, 2) > K(ix, pi) K(ix, p2). It easily follows that

2) By (C-1), K(ix, p2) K(2, 2) > 0. Thus

(o, 12 2 1-P)[G(2) G(ii)] < I~ )

By part 1) of the lemma, r (v, 2) TU 1 -~l

3) By (C-1), K(2, 1i) K(A pi) > 0. Thus

sup R(0, f t) + pl [G(2) G(ii)] < sup R(0, 2 -

BEe sEe

By part 1) of the lemma, supose R(8, 1) < supeoe R(8, 2 -

REFERENCES

[1] R. E. Kalman, "A new approach to linear filtering and prediction problems," ASIME

Transactions, Journal of Basic Engineering, vol. 82, pp. 34-45, Mar. 1960.

[2] J. Heffes, "The effect of erronous models on the Kalman filter response," IEEE Trans.

Automat. Contr, vol. AC-11, pp. 541-543, Apr. 1966.

[3] T. Nishimura, "On the a priori information in sequential estimation problems," IEEE Trans.

Automat. Contr, vol. AC-11, pp. 197-204, Apr. 1966.

[4] T. Nishimura, "Error bounds of continuous Kalman filters and the application to orbit

determination problems," IEEE Trans. Automat. Contr, vol. AC-12, pp. 268-275, Jun.

1967.

[5] M. Mintz, "A note on minimax estimation and Kalman filtering," IEEE Trans. Automat.

Contr, vol. AC-14, pp. 588-590, Oct. 1969.

[6] J. M. Morris, "The Kalman filter: A robust estimator for some classes of linear quadratic

problems," IEEE Trans. Inform. Theory, vol. IT-22, pp. 526-534, Sep. 1976.

[7] V. Poor and D. P. Looze, "Minimax state estimation for linear stochastic systems with noise

uncertainty," IEEE Trans. Automat. Contr, vol. AC-26, pp. 902-906, Aug. 1981.

[8] C. J. Martin and M. Mintz, "Robust filtering and prediction for linear systems with

uncertain dynamics: A game-theoretic approach," IEEE Trans. Automat. Contr, vol. AC-28,

pp. 888-896, Sep. 1983.

[9] S. Verdli and H. V. Poor, "On minimax robustness: A general approach and applications,"

IEEE Trans. Inform. Theory, vol. IT-30, pp. 328-340, Mar. 1984.

[10] S. Verdli and H. V. Poor, "Minimax linear observers and regulators for stochastic systems

with uncertain second-order statistics," IEEE Trans. Automant. Contr, vol. AC-29, pp.

499-511, Jun. 1984.

[11] J. C. Darragh and D. P. Looze, "Noncausal minimax linear state estimation for systems with

uncertain second-order statistics," IEEE Trans. Automat. Contr, vol. AC-29, pp. 555-557,

Jun. 1984.

[12] B. I. Anan'ev, "On minimax state estimates for multistage statistically uncertain systems,"

Problems of Control and Information Theory, vol. 18, pp. 27-41, 1989.

[13] Y. L. Chen and B. S. Chen, "Minimax robust deconvolution filters under stochastic

parametric and noise uncertainties," IEEE Trans. Signal Processing, vol. 42, pp. 32-45, Jan.

1994.

[14] B. I. Anan'ev, "Minimax estimation of statistically uncertain systems under the choice of a

feedback parameter," Journal oflMathematical Systems, Estimation, and Control, vol. 5, pp.

1-17, 1995.

[15] J. O. Berger, Statistical Decision Theory and Ba!
edition, 1985.

[16] J. L. Hodges, Jr. and E. L. Lehmann, "The use of previous experience in reaching statistical

decisions," Ann. 1Math. Stat., vol. 23, pp. 396-407, Sep. 1952.

[17] B. Efron and C. Morris, "Limiting the risk of Bayes and empirical Bayes estimators part

1: The Bayes case," I. Amer Statist. Assoc., vol. 66, pp. 807-815, Dec. 1971.

[18] P. J. Kempthomne, "Numerical specification of discrete least favorable prior distributions,"

SIAIM J Sci. Statist. Comput., vol. 8, pp. 171-184, Mar. 1987.

[19] P. J. Kempthomne, "Controlling risks under different loss functions: The compromise

decision problem," Ann. Statist., vol. 16, pp. 1594-1608, Dec. 1988.

[20] I. M. Johnstone, "On minimax estimation of a sparse normal mean vector," Ann. Statist.,

vol. 22, pp. 271-289, Mar. 1994.

[21] R. F. Berg, "Estimation and prediction for maneuvering target trajectories," IEEE Trans.

Automat. Contr, vol. AC-28, pp. 294-304, Mar. 1983.

[22] M. H. Kao and D. H. Eller, "Multiconfiguration Kalman filter design for high-performance

GPS navigation," IEEE Trans. Automat. Contr, vol. AC-28, pp. 304-314, Mar. 1983.

[23] L. Danyang and L. Xuanhuang, "Optimal state estimation without the requirement of a

priori statistics information of the initial state," IEE Trans. Automant. Contr, vol. 39, pp.

2087-2091, Oct. 1994.

[24] Y. Levinbook and T. F Wong, "State estimation with initial state uncertainty,"

IEEE Transactions on Information Theory, 2005, Submitted for publication. URL:

http://wireless.ece.ufl.edu/~twong/Preprinsmnmxpf

[25] A. Wald, Statistical Decision Functions, John Wiley and Sons, New York, 1950.

[ 26] T. S. Ferguson, 1Mathematical Statistics: A Decision Theoretic Approach, Academic Press,

New York, 1967.

[27] P. Billingsley, Convergence of Probability 1Measures, John Wiley & Sons, New York, 1968.

[28] W. Rudin, Principles of Mathematicalnrrtab \i\. McGraw-Hill, New York, 3rd edition,

1976.

[29] D. W. Stroock, Probability Theory, An Analytic View, Cambridge University Press, New

York, 1993.

[30] L. LeCam, "An extension of Wald's theory of statistical decision functions," Ann. 1Math.

Stat., vol. 26, pp. 69-81, Mar. 1955.

[31] H. Kudo, "On the property (W) of the class of statistical decision functions," Ann. 1Math.

Stat., vol. 37, pp. 1631-1642, Dec. 1966.

[32] M. Sion, "On general minimax theorems," Pacific J. 1Math., vol. 8, pp. 171-176, 1958.

[33] G. K. Pedersen, Arab\ \i\ Now, Springer-Verlag, New York, 1989.

[34] A. N. Shiryaev, Probability, Springer-Verlag, New York, 2nd edition, 1989.

[35] J. L. Doob, 1Measure Theory, Springer-Verlag, New York, 1994.

[36] W. Rudin, Functional Analysis, McGraw-Hill, New York, 2nd edition, 1991.

[37] E. L. Lehmann, Theory of Point Estimation, Wiley, New York, 1983.

[38] W. Nelson, "Minimax solution of statistical decision problems by iteration," Ann. 1Math.

Stat., vol. 37, pp. 1643-1657, Dec. 1966.

[39] J. Wolfowitz, "On e-complete class of decision functions," Ann. 1Math. Stat., vol. 22, pp.

461-465, Sep. 1951.

[40] C. D. Aliprantis and K. C. Border, Infinite DimensionalAnahn \i\.: A Hitchhiker's Guide,

Springer-Verlag, Berlin, 1994.

[41] N. M. Roy, "Extreme points of convex sets in infinite dimensional spaces," Amer 1Math.

1Monthly, vol. 94, pp. 409-422, May 1987.

[42] S. Boyd and L. Vandenberghe, Convex Optimization, Cambridge University Press,

Cambridge, 2004.

[43] L. Mirsky, "A trace inequality of John von Neumann," Maonashe~tfire flrMathematik, vol.

79, pp. 303-306, 1975.

[44] T. Kailath, A. H. Sayed, and B. Hassibi, Linear Estimation, Prentice Hall, NJ, 2000.

[ 45] S. S. Blackman, 1Multiple Target Tracking with RadarApplications, Artech House,

Washington, DC, 1986.

[46] D. G. Lainiotis, S. K. Park, and R. Krishnaiah, "Optimal state-vector estimation for non-

Gaussian initial state-vector," IEEE Trans. Automat. Contr, vol. AC-16, pp. 197-198, Apr.

1971.

BIOGRAPHICAL SKETCH

Yoav N. Levinbook was born in Tel Aviv, Israel, on October 30, 1974. He received the B.S.

degree Magnaa com laude) from Tel Aviv University, Israel, in 2000 and the M.S. and Ph.D.

degrees in electrical and computer engineering from the University of Florida, Gainesville, in

2006 and 2007, respectively.

He was with the Motorola Semiconductor, Herzliya, Israel, and Smartlink, Netanya,

Israel, as an electrical engineer. His research interests include statistical decision theory, state

estimation, signal processing for communications, and sensor networks.

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IwouldliketothankProfessorTanF.Wong,myadvisorandthechairofthesupervisorycommittee,forhisguidance,usefuladvice,and,inparticular,forthefreedomandencouragementhegavemeforpursuingmyownresearchinterests.IfeelthatIsignicantlyevolvedasanelectricalengineerinthefouryearsIworkedcloselywithhim.IhavenodoubtthatwithouthishelpIwouldnothavecompletedthiswork.Iwishtotakethisopportunitytothankallthemembersofthesupervisorycommitteefortheirhelpfulcommentsandsuggestionswhichhavehelpedimprovethiswork.IwouldalsoliketothankProfessorPaulRobinsonandProfessorSergeiShabanovfromtheDepartmentofMathematics,whoweremyinstructorsinseveralcourses.TheknowledgeIhavegainedfromthemprovedtobeveryvaluableforthiswork.Iwouldliketoexpressmydeepestgratitudetomybelovedmotherandlatefather,whichhavealwayssupportedmeandencouragedme.IhopeIliveduptotheirexpectations.Finally,Iamindebtedtomydearwife,Eliane,forhersupport,encouragement,andpatience.Withouther,Icouldnothaveconfrontedallthedifcultiesofthelastfouryears. 4

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page ACKNOWLEDGMENTS .................................... 4 LISTOFFIGURES ....................................... 7 LISTOFABBREVIATIONS .................................. 8 ABSTRACT ........................................... 9 CHAPTER 1INTRODUCTION .................................... 11 2GENERALNOTATIONANDCONVENTIONS .................... 17 3DECISIONTHEORETICFORMULATION ....................... 19 4GENERALDECISIONTHEORETICRESULTS .................... 24 5THECASETHATTHERISKISSPECIFIEDBYALOSSFUNCTION ........ 34 6THECASEOFCONVEXLOSSFUNCTION ...................... 40 7FINDINGAMINIMAXESTIMATORANDTHEDUALPROBLEM ......... 49 8APPROXIMATINGAMINIMAXESTIMATOR .................... 52 9THERESTRICTEDRISKBAYESPROBLEMASAMINIMAXPROBLEM .... 61 10ESTIMATIONWITHARESTRICTIONONTHEOBSERVATIONSTHATCANBEUSED ......................................... 66 11THESTATEESTIMATIONPROBLEM ......................... 73 12AFFINESTATEESTIMATIONBASEDONQUADRATICLOSSFUNCTIONS ... 77 12.1FindingaRestrictedRiskBayesSolution ..................... 77 12.2FindingaMaximizeroftheRisk .......................... 84 12.3ConnectiontotheKalmanFilterand-MinimaxApproach ............ 87 12.4NumericalExample ................................. 90 13STATEESTIMATIONWITHINITIALSTATEUNCERTAINTY ........... 96 13.1ConditionalMeanEstimators ............................ 97 13.2ApproximationstoMinimaxEstimators ...................... 98 13.3NumericalExample ................................. 100 14CONCLUSIONS ..................................... 106 APPENDIX 5

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................................. 107 BPROOFOFLEMMA9.1 ................................. 109 CPROOFOFLEMMA9.2 ................................. 110 REFERENCES ......................................... 111 BIOGRAPHICALSKETCH .................................. 114 6

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Figure page 12-1AchievedBayesriskvs.maximumrisk .......................... 94 12-2ThemaximumriskoveroftheBayes,minimax,andrestrictedriskBayessolu-tionsvs. 95 13-1AfullviewofXf0,whichisanite(;V)-densesubsetofX0 103 13-2Azoom-inviewofthebottomleftcornerofXf0,whichisanite(;V)-densesubsetofX0 104 13-3Theriskof^x(0),thederived-optimalestimator,asafunctionofx02Xf0 104 13-4Theaprioridistribution0,whichisdenedonXf0 105 13-5ThemaximumriskoftheKalmanFilterinitializedwithzeromeanandcovariance2Iasafunctionof 105 7

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CM:conditionalmean......................................97KF:KalmanFilter........................................11LMMSE:linearminimummeansquarederror.........................11MSE:meansquarederror....................................45 8

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1 ]isthelinearminimummeansquarederror(LMMSE)estimator.IfinadditionallthestochasticquantitiesareGaussian,theKFistheminimummeansquarederrorestimator.Sincetheassumptionofcompleteknowledgeoftheaprioridistributionisseldomsatised,aBayesianapproachisusedinpractice.Theaprioridistributionsoftheinitialstate,modelnoise,andmeasurementnoisearelearnedfrompastexperienceandusedasapproximationsofthecorrespondingtruedistributions.Nevertheless,evenifextensivepastexperienceisavailable,theestimateddistributionsmaystilldeviatefromthetrueones.TheeffectofsucherrorsintheaprioriinformationontheperformanceoftheKFisstudiedin[ 2 ][ 4 ].Theeffectoftheerrorsintheaprioriinformationoftheinitialstate,modelnoise,andmeasurementnoisemaybeverysignicantandaKFupdatedbasedonerroneousaprioriinformationmayperformpoorly.Thusitisnecessarytoconsiderotherapproachesthatarerobustagainstuncertaintiesintheaprioridistributionoftheinitialstate,modelnoise,andmeasurementnoise.Thestateestimationliteraturedealsextensivelywiththegeneralproblemoflinearsystemswithstochasticordeterministicuncertaintiesusinggametheoryandtheminimaxapproach(cf.[ 5 ][ 14 ]andthereferencestherein).Usuallytheso-called-minimaxapproachisadopted.The-minimaxapproach[ 15 ]regardstheparameterasrandomwithitsdistributionliesinaclass.However,theexactdistributionintheclassisunknown.A-minimaxestimatorisanestimatorthatminimizesthesupremumoftheBayesrisk,wherethesupremumistakenoverallelementsof.Whenthe-minimaxapproachisused,theclassofavailableestimatorsisusuallyrestricted 11

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16 ],isacompromisebetweentheBayesapproachandtheminimaxapproach.ArestrictedriskBayesestimatorminimizestheBayesriskwithrespecttoanaprioridistributionsuggestedbasedonsomepastexperiencesubjecttotherestrictionthatthemaximumriskdoesnotexceedtheminimaxriskbymorethanagivenamount.Thisapproachutilizesavailableaprioriinformationbutatthesametimeprovidesasafeguardincasethisinformationisnotaccurate.Iftheaprioriinformationisfairlyaccurate,arestrictedriskBayesestimatorhasgoodBayesriskproperties.OtherworkconsideringtherestrictedriskBayesapproachorcloselyrelatedapproachesinclude[ 17 ][ 20 ].DespitetheappealingformulationoftherestrictedriskBayesapproach,ithasnotbeenutilizedinthecontextofstateestimation.Althoughtheproblemofstateestimationwithstochasticuncertaintieshasbeenapproachedfromthe-minimaxapproach,webelievethatapproachingthisproblemfromtherestrictedriskBayesapproachalsohasaconsiderablemerit.Ifastateestimationproblemcanberegardedasazero-sumtwo-persongame(henceforthtobereferredasagame)againstarationalopponent,thenthe-minimaxapproachseemsveryattractive.However,inmostapplications,ifweregardthestateestimationproblemasagame,thegameisagainstNature.Usingthe-minimaxapproachinthiscasecorrespondstoaverypessimisticviewpointthatregardsNatureasarationalopponentthatwishestocauseusthelargestpossibleloss.The-minimaxapproachmaystillseemreasonableinthecasethatthereisnoaprioriinformationthatenablesustoregardcertaindistributionsinasmorelikelythanothers.However,inmanyapplications,someaprioriinformationregardingthetruedistributionmaybeavailable.Thisaprioriinformationmaybeintheformofanominaldistribution,whichissuggestedbasedonsomepastexperience.Itiswellknownthatundercertainconditions,a-minimaxestimatorisaKFrelativetoaleastfavorablea

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21 ]and[ 22 ]),itseemsmorereasonabletomodeltheinitialstatevectorasdeterministicandunknown.OneexamplethatfallswithintheclassicalestimationframeworkisDanyangandXuanhuang[ 23 ],wherethestateestimationproblemwasconsideredfromaleastsquaresviewpointandthebestlinearunbiasedestimatorwasderived.In[ 24 ],theauthorsconsidertheproblemofstateestimationwithinitialstateuncertaintyfromadecisiontheoreticpointofview.Theinitialstatevectorisregardedasdeterministicandunknown.Itisonlyknownthattheinitialstatevectorbelongstoaparameterset.Therisk,basedonquadraticlossfunctions,isconsideredastheperformancemeasure.Thesearchforaminimaxestimatorisdonewithintheclassofallpossibleestimators.Minimaxestimatorsarederivedforthecaseofunboundedparametersetandapproximationsofminimaxestimatorsarederivedforthecaseofboundedparameterset.Inthiswork,wewillrepeatsome 14

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24 ]thatdealwiththeboundedparametersetcaseinordertoillustratehowminimaxestimatorscanbeapproximatedwitharbitrarilyprescribedaccuracy.Whilewearemainlyinterestedinthestateestimationproblem,alargepartofthisworkwillbeconcernedwithamoregeneralestimationproblem.Infact,someoftheexistenceresultssuchastheexistenceofaminimaxestimatorandarestrictedriskBayessolutionholdinaverygeneralsettingandmayhaveapplicabilitynotonlyinthestateestimationproblem.Therestofthisworkisorganizedasfollows.InChapter 2 ,wepresentnotationandconventionsthatareusedthroughoutthiswork.InChapter 3 ,wepresentageneraldecisiontheoreticformulationthatisneededinordertoaddresstheproblemofstateestimationwithstochasticanddeterministicuncertaintiesandalsotoderiveother,moregeneral,results.InChapter 4 ,wederiveseveralgeneraldecisiontheoreticresults,whicharebasedonwellknownresultsfromdecisiontheoryandgametheory;theapplicabilityoftheseresultsisnotlimitedonlytotheproblemofstateestimationwithstochasticanddeterministicuncertainties.InChapter 5 ,weconsiderthecasethattheriskfunctionisspeciedbyalossfunction.WederiveratherweakconditionsthatguaranteetheexistenceofaminimaxestimatorandarestrictedriskBayessolution.InChapter 6 ,werestrictourselvestoconvexlossfunctions,ingeneral,andthequadraticlossfunction,inparticular.InChapter 7 ,wediscusshowaminimaxestimatorcanbefoundbysolvingthedualproblemofndingaleastfavorableaprioridistribution.Since,ingeneral,ndingaminimaxestimatormaybeanextremelydifculttask,inChapter 8 wediscusshowonecanderiveapproximationstominimaxestimators,wheretheapproximationcanbemadeasaccurateasdesired.InChapter 9 ,weconsiderthegeneralrestrictedriskBayesestimationproblemandshowthatthisproblemisequivalenttoasequenceofminimaxestimationproblems.Insomeestimationproblemstherearerestrictionsontheobservationsthatcanbeusedinordertoestimatetheparameters.Thisisthecaseinthestateestimationprobleminwhicheachstatecanbeestimatedusingonlycertainobservations.WeconsiderthistypeofestimationproblemsinChapter 10 .InChapters 11 12 ,and 13 ,werestrictourselvestotheproblemofstateestimationwithuncertaintiesintheinitialstate,modelnoise,andmeasurementnoise,which 15

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11 ,wederivesomegeneralexistenceresultsthatarebasedontheresultsofthepreviouschapters.InChapter 12 ,weconsiderthecaseofstochasticuncertaintiesintheinitialstate,modelnoise,andmeasurementnoise,andrestrictourselvestoafneestimators.WeproposeamethodthatcanbeeasilyusedtoderivearestrictedriskBayessolutioninmanyimportantcases.InChapter 13 ,weconsiderthecaseofdeterministicinitialstateuncertainty,andsearchforestimatorswithintheclassofallestimators.WeconcludethisworkinChapter 14 16

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25 ]ofthestatisticaldecisionproblemasazerosumtwo-persongame,Naturechoosesanelement2,calledthetruestateofnature.Theexperimenterneedstoreachacertaindecisionbasedontheobservationywithoutknowingthetruestateofnature.Theexperimenterreachessuchadecisionbychoosingadecisionrulefromaclassofdecisionrules.Herewearemainlyinterestedinestimationproblems,althoughsomeoftheresultsofthisworkapplyalsotodecisionproblems.Inclassicalestimation,theexperimenterestimatesthetruestateofnature2ormoregenerallyafunctionof.Inthiscasethespace(;F;Q)canbetakentobe(Y;FY;Q),whereQisthedistributionofY.Inthiswork,weareinterestedinamoregeneralcase.Supposethereisgivenanadditionalmeasurablespace(X;FX)andarandomelementX:!(X;FX).SupposeYandXhaveajointdistributiondenedontheproductmeasurablespace(YX;FYFX).SupposetheexperimenterwantstoestimatethevaluextakenbyX.Inthiscase,thespace(;F;Q)canbetakentobethespace(YX;FYFX;Q),whereQisthejointdistributionofYandXandisanelementofP.Notethatthepreviousmentionedcasecanberegardedasaspecialcase,inwhichthedistributionofXassignsmass1tothetruestateofnature2.SupposetheexperimenterisallowedtochooseestimatesforxfromaclassD.TheclassD,equippedwithatopology,isreferredtoasthespaceofpossibleestimates.OftenDcoincides 19

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26 ]and[ 25 ],withslightmodications.Thesedenitionsaregivenforanestimationproblem(;X;R).

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3.3 canbeweakenedtotheassumptionthatinf2inf^x2XR(;^x)>,butforthesakeofpresentationitisadvantageoustoassumethatRisnonnegative.Weneedadditionaldenitions.If^xissuchthatR(;^x)isboundedfrombelowandisaB()-measurablefunction,wedenotetheBayesriskrelativeto2Mof^x r(;^x)=ZR(;^x)d: Inthiswork,weconsideronlyestimationproblems(;X;R)forwhichR(;^x)isboundedfrombelowandisaB()-measurablefunctionforall^x2X.HencetheBayesriskisalwaysdened.

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3.1 ,inf^x2Xr(;^x)<+1,andthetermr(i;^x)inf^x2Xr(i;^x),intheabovedenition,iswelldened.

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26 ,Exercise2.2.1]) ConsiderthespaceMequippedwiththetopologyofweakconvergence[ 27 ,pp.236].ThetopologyofweakconvergencemakesMaHausdorffspace. 1. 2. Let^x2XbesuchthatR(;^x)<+1foreach2andR(;^x)iscontinuouson.Thenr(;^x)iscontinuousonM. 3. Let^x2XbesuchthatR(;^x)<+1foreach2andR(;^x)isuppersemicontinu-ouson.Thenr(;^x)isuppersemicontinuousonM. Proof. 28 ,Exercise2.25]).ThisimpliesthatMismetrizable[ 29 ,pp.122].Inaddition,[ 29 ,Theorem3.1.9]furnishesthatMiscompact.2)By[ 29 ,Theorem3.1.5],Rfdisacontinuousfunctionofforanyboundedandcontinuousf.SinceR(;^x)isacontinuousreal-valuedfunctiononacompactset,itisbounded.Hencer(;^x)iscontinuousonM.3)By[ 29 ,Theorem3.1.5],Rfdisanuppersemicontinuousfunctionofforanyup-persemicontinuousfthatisboundedfromabove.SinceR(;^x)isanuppersemicontinuousreal-valuedfunctiononacompactset,itisboundedfromabove.Hencer(;^x)isuppersemicon-tinuousonMforany^x2C. 24

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30 ]. 25

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31 ].SinceU(M)iscompact,R(A;M)isrelativelycompact.Thusthereexistsasubnetf^xbgb2BthatconvergestoapointuintheclosureofR(A;M).HenceliminfR(;^xb)=limR(;^xb)=u.SinceR(A;M)ishalf-closed,thereexistsanelementv2R(A;M)suchthatvu.Hencethereexistsanelement^x2XsuchthatR(;^x)liminfR(;^xb).Supposeforeachnetf^xaga2AinA,thereexistsasubnetf^xbgb2Bandanelement^x2Asuchthatliminfr(;^xb)r(;^x).LetubelongtotheclosureofR(A;M).Thenthereexistsanetfuaga2A2R(A;M)thatconvergestou.Clearlyforeachelementuainthisnet,thereisanelement^xa2Asuchthatua=R(;^xa).ThuslimR(;^xa)=u.Itfollowsthatthereexistsasubnetf^xbgb2Bandanelement^x2AsuchthatliminfR(;^xb)R(;^x),whenceuR(;^x)andR(A;M)ishalf-closed.Inentirelyanalogousway,Ahastheproperty(W)ifandonlyifforeachnetf^xaga2AinA,thereexistsasubnetf^xbgb2Bandanelement^x2AsuchthatliminfR(;^xb)R(;^x).SinceeachcanbeidentiedasanelementinM,asdiscussedpreviously,itisclearthattheproperty(W)impliestheproperty(W).Theproperty(W),asformulatedwithnets,iscloselyrelatedtoWald'sweakcompactness[ 25 ,pp.53].Theonlydifferenceisthatinthecurrentdenitionsequencesarereplacedbynets.Hencetheproperty(W)isweakerthanWald'sweakcompactness.SufcientconditionsforasubsetAofXtohavetheproperty(W)aregivenin[ 31 ].AsimplesufcientconditionisthatthereexistsaHausdorfftopologyJforAsuchthatAiscompactandR(;)islowersemicontinuousonAforeach2.Similarly,asufcientconditionforAtohavetheproperty(W)isthatthereexistsaHausdorfftopologyJforAsuchthatAiscompactandr(;)islowersemicontinuousonAforeach2M.SupposethereexistsatopologyJforAsuchthatAiscompact.Thenifthetopologicalspace(A;J)satisestherstaxiomofcountability,byFatou'sLemma,therequirementthatr(;)isalowersemicontinuousonAforeach2McanbereplacedbytherequirementthatR(;)isalowersemicontinuousonAforeach2. 26

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31 ]. Proof. 25 ].Themaindifferencesisthatsequencesarereplacedbynets.RecallthatweassumethatV(;X;R)<+1.Letfxngbeasequencesuchthatsup2R(;^xn)convergestoinf^x2Xsup2R(;^x).Thenthereexistsasubnetf^xaga2Aandanelement^x2XsuchthatliminfR(;^xa)R(;^x).Sincef^xaga2Aisasubnetoffxng,limsupn!1R(;^xn)liminfR(;^xa).Inaddition,limn!1sup2R(;^xn)limsupn!1R(;^xn)forall2.Thusinf^x2Xsup2R(;^x)sup2R(;^x),and^xisaminimaxestimator. Afteraddressingtheexistenceofaminimaxestimator,wewanttoaddresstheexistenceofarestrictedriskBayessolution. Proof. 4.1 ,thereexistsaminimaxestimator.Letf^xngbeasequenceinX(C0;R)suchthatlimn!1r(;^xn)=inf^x2X(C0;R)r(;^x).ThensinceXhastheproperty(W),thereexistsasubnetf^xaga2Aoffxngandanelement^x2Xsuchthatliminfr(;^xa)r(;^x)foreach2M.Certainlyr(;^xn)C0forallnandeach2M.Sincef^xaga2Aisasubnetoffxng,itisclearthatliminfr(;^xa)C0

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PuttingC0=+1intheabovetheorem,wegetthatforeach2M,thereexistsaBayessolutionrelativeto(providedthatXhastheproperty(W)).ThefollowingTheorem,aversionofWald'scompleteclasstheorem,appearsin[ 31 ]. 32 ,Theorem4.2]. Proof. 4.1 ,Miscompactandr(;^x)isuppersemicontinuousonMforeach^x2X.ItiseasytoverifythatsinceXissubconvex,foreach^x0;^x002Xand0<<1,thereexists^x2Xsuchthatr(;^x)r(;^x0)+(1)r(;^x00)forall2.Certainlyr(;^x)isconcaveonMforall^x2X.Applying[ 32 ,Theorem4.2],inf^x2Xsup2Mr(;^x)=sup2Minf^x2Xr(;^x):Thus(;X;R)isstrictlydetermined.Sinceinf^x2Xr(;^x)isuppersemicontinuousonacompactsetM,thereexists02Msuchthatinf^x2Xr(0;^x)=sup2Minf^x2Xr(;^x):Thus0isaleastfavorableaprioridistribution. 28

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4.4 thatR(;^x)<+1foreach^x2XandR(;^x)isuppersemicontinuousonforeach^x2Xisobviouslytoostrong.Inthefollowingtheoremthisassumptionisconsiderablyweakened,butatthepriceofastrongerassumptiononX.LetBXdenotetheclassofBayessolutionsrelativeto2M.Beforestatingthetheorem,weneedthefollowinglemma. 4.2 itisshownthatifXhastheproperty(W),thereexistsaBayessolutionrelativetoforall2M.Thelemmafollowseasily. Proof. 32 ,Theorem4.2], Fix2M.Ononehand,sinceR(B;M)GB,infg2GBg()infg2R(B;M)g()=inf^x2Br(;^x):Ontheotherhand,ifwexg2GB,thenthereexistanintegern>0,realnumbers1;:::;n>0,andelementsu1;:::;un2R(B;M)suchthatg=Pni=1iuiandPni=1i=1.Let1inbesuchthatui()uj()forj=1;:::;n.Theng()ui(). 29

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SinceGGB, Fixg2G.ThensinceXissubconvex,thereexists^x2Xsuchthatr(;^x)g()forall2M.Hence By( 4 )( 4 )andLemma 4.2 ,(;X;R)isstrictlydetermined.Sinceinfg2GBgisuppersemicontinuousonthecompactsetM,thereexists02Msuchthatinfg2GBg(0)=sup2Minfg2GBg():By( 4 )andLemma 4.2 ,0isaleastfavorableaprioridistribution. Ifthecompactnessofisdroppedintheabovetheorem,thentheremaynotbealeastfavorableaprioridistribution.Ifthereexistsnoleastfavorableaprioridistribution,buttheestimationproblem(;X;R)isstrictlydetermined,aminimaxestimatorcanbefoundasaBayessolutionrelativetoaleastfavorablesequenceofaprioridistributions,i.e.,asequencefng2Mthatsatiseslimn!1inf^x2Xr(n;^x)=sup2Minf^x2Xr(;^x).Wearenotgoingtodealwiththequestionofhowsuchasequencecanbeconstructed.ThefollowingtheoremisessentiallyWald'sTheorem3.9in[ 25 ]. 30

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Proof. 25 ,Theorem3.9]applieswithoutanychanges. Wheniscompact,wehavethefollowingversiontoWald'scompleteclasstheorem. Proof. ByLemma 4.1 part1),thereexists02Mandasubsequencefikg2Msuchthatikconvergesweaklyto0.Bythehypothesisofthetheorem,r(;^x)iscontinuousonMforany^x2B.Thusinf^x2Br(;^x)isuppersemicontinuousonM[ 33 ,Proposition1.5.12].ByLemma 4.2 ,inf^x2Xr(;^x)isuppersemicontinuousonM.Sincer(;^x)islowersemicontinuousonM,inf^x2Xr(;^x)r(;^x)isuppersemicontinuousonMand By( 4 )and( 4 ),inf^x2Xr(0;^x)r(0;^x),whence^xisaBayessolutionrelativeto0.

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4.1 and 4.2 hold.ThenXhastheproperty(W),thereexistsaminimaxestimator,andthereexistsarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;X;R).If,inaddition,iscompactandCondition 4.3 holds,(;X;R)isstrictlydetermined,thereexistsaleastfavorableaprioridistribution02M,anyminimaxestimatorisaBayessolutionrelativeto0,andtheclassofBayessolutionsisessentiallycomplete.If,inaddition,Condition 4.4 holds,thenthe(essentiallyunique)Bayes

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Proof. 25 ]).Forthesakeofsimplicity,weassumethat(;X;R)isanestimationproblem.TheproofcanbetriviallymodiedtothecasethatXisnotaclassofestimators,butanarbitraryset.Letr(;a)=RR(;a)d,where2Manda2X.ItcanbeveriedthatsinceXisacompactmetrizablespaceandR(;)islowersemicontinuousonXforeach2,fr(;a):a2Xgishalf-closed,i.e.,Xhastheproperty(W).Itcanbeveriedthatsince(X;R)(X;R),Xhastheproperty(W).ByTheorem 4.1 ,thereexistsaminimaxestimator.ByTheorem 4.2 ,thereexistsarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;X;R).If,inaddition,Condition 4.3 holds,itfollowsfromtheprecedingresults,that(;X;R)isstrictlydetermined,thereexistsaleastfavorableaprioridistribution02M,anyminimaxestimatorisaBayessolutionrelativeto0,andtheclassofBayessolutionsisessentiallycomplete.SupposeCondition 4.4 holdsaswell.Let0denotealeastfavorableaprioridistribution.SinceanyminimaxestimatorisaBayessolutionrelativeto0(Theorem 4.6 ),the(essentiallyunique)Bayessolutionrelativeto0isanessentiallyuniqueminimaxestimator.By[ 26 ,Theorem2.3.1],thisBayessolutionisadmissible. WhilesometimesitispossibletoverifyConditions 4.1 4.4 directly,othertimes,especiallywhenthereisnoclosedfromexpressionforR,itmaybedifculttocheckwhethertheseconditionshold.Inthenextchapter,weconsiderthecasethatRisbasedonalossfunctionL.Inthiscase,itispossibletoderiveconditionsthatcanbemoreeasilycheckedwhenthereisnoclosedformexpressionforR. 33

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WeimposeseveralconditiononthefamilyP,thespaceY,thespaceDofpossibleesti-mates,andthelossfunctionL. 34

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34 ,pp.220].If~gisaversionofPr(AjX),thenaccordingto[ 34 ,pp.221],aconditionalprobabilityoftheeventAgiventhatX=x,g(x),canbeconstructedasfollows:g(X(!))=~g(!)(i.e.,g(x)=~g(!),where!isanelementinsuchthatX(!)=x).SinceinthisworkthedistributionQisunknown,butisknowntobeanelementofP,theconditionalprobabilityofA2FgivenXandtheconditionalprobabilityofAgivenX=xdependonthetruestateofnature2. 1. Foreach!2,Q(jX)(!)isaprobabilitymeasureon(Y;FY). 2. ForeachA2FY,Q(AjX)isaversionoftheconditionalprobabilityPr(AXjX).Fix2.Condition 5.1 furnishesthatthereexistsaregularconditionaldistributionofYgivenXwhenQ=P[ 34 ,Theorem2.7.5].LetP(jX)denotearegularconditionaldistributionofYgivenXwhenQ=P.Foreachx2XandA2FY,wedeneP(Ajx)=P(AjX)(!),where!2issuchthatX(!)=x.WecallP(jx)aregularconditionaldistributionofYgivenX=xwhenQ=P(oristhetruestateofnature).NotethatsinceP(jX)isregular,P(jx)isaprobabilitymeasureforeachx2X.Inaddition,foreachA2FY,P(Ajx)isaversionoftheconditionalprobabilityPr(AXjX=x).LetPYjX=fP(jx):x2X;2g. 5.3 and 5.4 wereaddedtoguaranteethattheintegrationsnecessaryinthecalculationoftheriskfunctionarewelldened.ItcanbeveriedthatifConditions 5.1 5.4 hold,R(;^x)isanextendedreal-valuednonnegativeB()-measurablefunctionandhencethe 35

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30 ].Itisusefultodiscussthepropertiesofthisvectorspace.Thefollowingdiscussionessentiallyappearsin[ 30 ]and[ 31 ].LetCc(D)denotetheclassofcontinuousreal-valuedfunctionsonDwithcompactsupport,andletjjujj=supd2Dju(d)jforu2CC(D).LetL1denotetheBanachspaceofequivalenceclassesofintegrablefunctionson(Y;FY;)withnormjjfjj1=RYjfjd.DenotebyLPYjXthelinearsubspaceofL1spannedbyPYjX.LettheproductspaceCC(D)LPYjXbeequippedwiththenormjj(u;f)jj=jjujj_jjfjj1foru2Cc(D)andf2LPYjX.LetdenotethevectorspaceofboundedlinearfunctionalsonCC(D)LPYjX.Theweak-topologyturnsintoalocallyconvextopologicalvectorspace[ 33 ].Callg1;g22m(FY)PYjX-equivalentifRYjg1g2jfd=0forallf2PYjX.Calltwoestimators^x1;^x22DPYjX-equivalentifforeachD02B(D),^x1(D0j)and^x2(D0j)arePYjX-equivalent.Let~Ddenotetheclassofequivalenceclassessoobtained.Afunctional2issaidtobepositiveifu0andf0imply(u;f)0.AccordingtoLeCamifConditions 5.1 5.5 hold,everypositivelinearfunctionalofnorm1canberepresentedbyan 36

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5.1 5.5 and 5.7 hold.Thenthetopologicalspace(D;J)ismetrizableandR(;)islowersemicontinuousonDforall2.If,inaddition,Discompact,Discompact. Proof. 5.5 ,thespaceCc(D)isseparable[ 30 ].SincebyCondition 5.1 ,Yisaseparablemetricspace,thespaceL1isseparable[ 35 ,pp.92].SinceLPYjXisasubspaceofaseparablenormedspace,itisalsoseparable.ItfollowsthatthespaceCc(D)LPYjXisseparable.ByTheorem[ 36 ,Theorem3.16],if0isweak-compact,then0ismetrizable.ByBanach-AlaogluTheorem[ 33 ,Theorem2.5.2],thesetB=f2:jjjj1gisweak-compact,wherejjjjdenotestheoperatornormof2.ThusBismetrizable.SinceJistherelativetopology,Dismetrizable.LetR(x;;^x)=RYL(x;^x)p(yjx)d.Thenby( 5 ),R(;^x)=RXR(x;;^x)dPX(x).UsingtheresultsofLeCam[ 30 ],itcanbeshownthatR(x;;)islowersemicontinuousonDforeach(;x)2X.SinceDismetrizable,wehavebyFatou'sLemmathatR(;)islowersemicontinuousonDforeach2.SupposeDiscompact.LeCam[ 30 ]showedthataclassAofestimatorsisJ-compactifitisJ-closedandifthefollowingconditionsholds:Foreach>0andeach(;x)2X,thereexistsau2Cc(D)satisfying0u1andRYu(d)d^x(djy)dP(yjx)1uniformlyforall^x2A.TheprecedingconditioncertainlyholdsforDwhenDiscompact.SinceDisJ-closed,itiscompact. 37

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4.2 holdsunderveryweakconditions. Proof. A 5.1 5.7 hold.ThenCondition 4.1 and 4.2 holdfor(;D;R).Asaconsequence,thereexistsaminimaxestimatorandarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;D;R). Proof. 4.1 holds.LetusshowthatCondition 4.2 holds.SupposeDiscompact.ThenbyLemma 5.1 ,Condition 4.2 holds.SupposeDisnotcompact.Wewillusetheone-pointcompacticationofDtoprovethetheorem.Theideatousetheone-pointcompacticationoftheclassofpossibleestimatestoproveresultsofthetypeofthistheoremseemstoappearrstin[ 30 ].LetDdenotetheone-pointcompacticationofD,andlet12DdenotethepointthatisaddedtoD.LetL:XD!R[f+1gbedenedasfollows:Foreachx2X,L(x;d)=L(x;d)ifd2D,andL(x;1)=supd2DL(x;d).LetDdenotetheclassofallestimatorswithDastheirspaceofpossibleestimates.ByLemma 5.2 ,L2m(FXB(D))andDiscompact.LetL(x;^x)(y)=RDL(x;d)d^x(djy), 38

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5.1 5.5 and 5.7 holdfor(;D;R).ThenbyLemma 5.1 ,DiscompactandR(;)islowersemicontinuousonDforeach2.ItcanalsobeveriedthatR(;a)2m(B())foreacha2X.LetD(D)denotetheclassofestimators^xinDsuchthat^x(Djy)=1forally2Y.Weclaimthat(D(D);R)(D;R).Indeed,x^x02Dsuchthat^x0(1jy)>0forsomey2Y.LetY0=fy2Y:^x0(Djy)=0g.ClearlyY0ismeasurable.Let^x(Ajy)=^x0(A\Djy)=^x0(Djy)fory=2Y0(A2B(D)),andlet^x(jy)beaDiracmeasurewithrespecttoapointd02Dforally2Y0.Itcanbeveriedthat^x(D0jy)isB(Y)-measurableforeachD02B(D).Nowify2Y0,thenL(x;^x)=L(x;d0)L(x;1).Ify=2Y0,thenRDL(x;d)d^x(djy)=RDL(x;d)=^x0(Djy)d^x0(djy)x0(Djy)RDL(x;d)=^x0(Djy)d^x0(djy)+(1^x0(Djy))L(x;1)=RDL(x;d)d^x0(djy).ThusL(x;^x)L(x;^x0).Itfollowsthat^xisasgoodas^x0.Thisprovesthat(D(D);R)(D;R).SinceD(D)D,wehavethat(D(D);R)(D;R).Clearly(D;R)(D(D);R).Thus(D;R)(D;R).ItfollowsthatCondition 4.2 holds. AsaconsequenceofTheorem 5.1 ,undertheratherweakConditions 5.1 5.7 ,thereexistsaminimaxestimatorandarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;D;R).Inordertogetthestrongerresultswheniscompact,namelythat(;D;R)isstrictlydeterminedandthatthereexistsaleastfavorableaprioridistribution,weneedtoprovethatCondition 4.3 holds.Unfortunately,thisseemstorequireratherstrongconditionsonthelossfunctionandfamilyP.AsetofsuchconditionsiswellknownforthecasethatthelossfunctionisuniformlyboundedandPisdominatedbya-nitemeasure.However,wearemainlyinterestedinthecasethatthelossfunctionisunbounded(e.g.,thequadraticlossfunction).Moreover,inmanycasesPisnotnecessarilydominatedbya-nitemeasure. 39

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37 ,Theorem1.6.4]. 5.3 holds.ThenifConditions 6.1 and 6.2 hold,DD. Proof. 40

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Integrating( 6 )withrespecttoP,wehaveR(;^x)R(;^x0).Since^x02D,DD. SinceDD,weconsiderintherestofthischaptertheestimationproblem(;D;R)insteadoftheestimationproblem(;D;R).Clearly,DissubconvexbyJenseninequality.HenceCondition 4.1 holdsfor(;D;R).SinceDD,DD.HenceitisclearthatCondition 4.2 holdsfor(;D;R)ifitholdsfor(;D;R).ThusifConditions 5.1 5.7 hold,Condition 4.2 holdsfor(;D;R).InLemma 6.2 below,weshowthatunderweakconditions,Condition 4.4 alsoholdsfor(;D;R).InordertoprovethatCondition 4.3 holdsfor(;D;R),itseemsnecessarytomakeadditionalassumptionsonthelossfunctionandthefamilyP.Sometimesitisconvenienttorestricttheclassofestimatorsthatareavailabletotheexperi-mentertotheclassofafneestimators.Anestimator^xissaidtobeafneifitisnonrandomizedandisanafnefunctiononY.SinceweconsideronlythecasethatY=RNyandX=RNx,^xisafneexactlywhen^x=Ay+bforsomeA2RNxNyandb2RNx,andthespaceofpossiblees-timatesDisthenRNx.LetLdenotetheclassofafneestimators.ThespaceLcanbeidentiedwiththespaceRNxNyRNxwhere(A;b)2RNxNyRNxistheestimator^x=Ay+b2Landviceversa.ThusthespaceLcanbeidentiedwithanite-dimensionalvectorspacewiththefollowingadditionandmultiplicationbyascalar:If^x=(A;b),^x0=(A0;b0)andisascalar,^x+^x0=(A+A0;b+b0)and^x=(A;b).LetthespaceLbeequippedwiththenormjj^x^x0jjL=jjAA0jj2+jbb0j,where^x=(A;b)and^x0=(A0;b0).ClearlyLisconvex.ItiseasytoseethatJensen'sinequalityfurnishesthatR(;)isconvexoverLforeach2ifL(x;)isconvexoverDforeachx2X.ThusifL(x;)isconvexoverDforeachx2X,L

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4.1 holdsfor(;L;R).InTheorem 6.1 below,weshowthatunderratherweakconditions,Condition 4.2 holdsfor(;L;R).Asintheestimationproblem(;D;R),itseemsnecessarytomakefurtherassumptionsonthelossfunctionandthefamilyPinordertoprovethatCondition 4.3 holdsfor(;L;R).InLemma 6.2 below,weshowthatunderweakconditions,Condition 4.4 alsoholdsfor(;L;R). 5.3 5.4 5.6 ,and 5.7 hold.ThenCondition 4.2 holdsfor(;L;R).Asaconsequence,thereexistsaminimaxestimatorandarestrictedriskBayessolutionrelativeto(C0;)foreachC0V(;D;R)and2M. Proof. 5.7 ,liminfn!1L(x;^xn(y))L(x;^x(y))foreachy2Y.ByFatou'slemma,liminfn!1R(;^xn)R(;^x)foreach2.ThusR(;)islowersemicontinuousonLforeach2.LetusshowthatforeachsequenceofcompactsubsetsCnofLsuchthat[1n=1Cn=Landeachelement^xn=2Cn(n=1;2;:::),liminfn!1R(;^xn)=sup^x2LR(;^x)forall2.FixasequenceofcompactsubsetsCnofLsuchthat[1n=1Cn=Landasequencef^xng2Lsuchthat^xn=2Cn(n=1;2;:::).Fix2.Certainlyliminfn!1R(;^xn)sup^x2LR(;^x).Thusitislefttoprovethatliminfn!1R(;^xn)sup^x2LR(;^x).ByFatou'slemma, Fix(y;x)2YX.LetDn=f^xn(y):^xn2Cng(n=1;2;:::).WeclaimthatDnisacompactsubsetofD.Indeed,letfdigbeasequenceinDn.Thenthereexistsasequencef^x0iginCnsuchthat^x0i(y)=di.SinceCniscompact,thereexistsasubsequencef^x0ijgofthesequencef^x0igandanelement^x02Cnsuchthat^x0ij!^x0.Thisimpliesthat^x0ij(y)!^x0(y),whencefdijg!^x0(y).Since^x0(y)2Dn,Dniscompact.Weclaimthat[1n=1Dn=D.Fixd2D.Clearlythereexists^x2Lsuchthat^x(y)=d(e.g.,^x=(A;b),whereA=0andb=d).Since[1n=1Cn=C,^x2Cnfornsufcientlylarge.Thisimpliesthatd2Dnfornsufciently 42

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5.6 ,liminfn!1L(x;dn)=supd2DL(x;d),wheredn=^xn(y).Itiseasytoverifythatsupd2DL(x;d)=sup^x2LL(x;^x(y)).Thusforeach^x2Lwehave Sinceyandxarearbitrary, Since^xisarbitrary, SinceLisanitedimensionalnormedspace,itislocallycompact,-compact,andmetrizable.LetLdenotetheone-pointcompacticationofLandlet1denotetheaddedpoint.Foreach2letR(;^x)=R(;^x)if^x2L,andletR(;1)=sup^x2LR(;^x).ByLemma 5.2 ,Liscompactandmetrizable,R(;)islowersemicontinuousonLforeach2,andR(;a)2m(B())foreacha2L.ClearlyLL.Since1istheonlyelementinLnLandR(;1)R(;^x)foreach^x2L,(L;R)(L;R).SinceLL,(L;R)(L;R).ThusCondition 4.2 holds. ThefollowinglemmaisconcernedwiththeessentialuniquenessofBayessolutions. 5.3 holds,andV(;X;R)<+1.ThenifCondition 6.2 holds,aBayessolutionrelativeto2MisanessentiallyuniqueBayessolutionrelativeto. Proof. 43

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Intherestofthischapter,weassumethelossfunctionisquadratic,i.e.,L:(x;d)7!jV(xd)j2,whereV2RNxNx.WeassumeVTV>0.TheextensiontothecaseVTV0isdiscussedlater.InthiscaseitcanbeveriedthatConditions 5.1 5.3 5.5 5.7 ,and 6.1 hold.ThusifDisconvexandCondition 6.2 holds,DD.CertainlyDissubconvexandCondition 4.1 holds.Inaddition,ifConditions 5.2 and 5.4 hold,Conditions 4.2 holdsfor(;D;R).Similarly,Condition 4.1 holdsfor(;L;R),andifConditions 5.2 and 5.4 hold,Condition 4.2 holdsfor(;L;R).Suppose,inaddition,thatPisaGaussianfamilyofdistributions,i.e.,YandXarejointlyGaussian,whenisthetruestateofnature,foreach2.ThenPYandPYjXarealsoGaussianfamiliesofdistributions.SupposethefamilyPYjXisdominatedbytheLebesgue-Borelmeasureon(RNyB(RNy)),whichisdenoted.ItcanbeeasilyveriedthatPYisalsodominatedbyandthatsince,inaddition,foreach2,p(y),thedensityofPwithrespectto,ispositive,themeasuresfPY:2garemutuallyabsolutelycontinuous.ThusCondition 6.2 holds.WewouldliketocheckunderwhatconditionsCondition 4.3 holds.Underthecurrentassumptions,itiswellknownthatthereexistsaregularconditionaldistributionofXgivenY=y,whenisthetruestateofnature.LetP(jy)denotethisconditionaldistribution,whichiswellknowntobeGaussian.Let^x:Y7!RNx;y7!E(Xjy),whereE(Xjy)

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InwhatfollowsBDistheclassofBayessolutionswhen(;D;R)isconsidered,i.e.,foreach^x2BD,thereexists2Msuchthatr(^x;)=inf^x2Dr(^x;).SinceDD,eachelementinBDisalsoaBayessolutionswhen(;D;R)isconsidered.Inwhatfollows,continuityoffunctionsfromintoRNandRNNismeantinthesenseoftheEuclideannormand2-norm,respectively. 4.3 holdsfor(;D;R). Proof. 6 )thatR(;^x)islowersemicontinuousonforeach^x2D. 45

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6 )that ItiseasytoverifythatR(0;^x)<+1.Puth(y)=(c1jyj+c2)2.Since0isarbitraryR(;^x)<+1foreach2.Moreover,foreach2,jV(^x(y)^x(y))j2h(y)andhis 46

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Letfngbesequencethatconvergesto0.Weshowedthatlimn!1jn0j=0.Thus BytheLebesguedominatedconvergencetheorem, Certainlyforeachn>0,jV(^xn(y)^x(y))j2h(y).SincehisP0-integrableand7!p(y)iscontinuousonforeachy2Y,awellknowntheoremonexponentialfamilies[ 37 ,Theorem1.4.1]furnishesthatZYh(y)pn(y)d!ZYh(y)p0(y)d:Itcanbeveriedthatsince7!p(y)iscontinuousonforeachy2Y,theaboveequationimpliesthatZYh(y)jpn(y)p0(y)jd!0:SincejV(^xn(y)^x(y))j2h(y), By( 6 )( 6 ),R(;^x)iscontinuouson.SinceR(;^x)<+1andR(;^x)iscontinuouson,whichisacompactset,R(;^x)isboundedon.ItfollowsthatR(;^x)isboundedandcontinuousonforeach^x2BD. 47

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4.3 holdsfor(;L;R). Proof. 6.2 andisomitted. WhiletheproofsofsomeoftheresultsofthischapterclearlybreakdownifVTVisnotpositivedenite,alltheseresultsarevalidalsointhecasethatVTVisnotpositivedenite.Thereisasimplemethodtoshowthatthisisindeedthecase.NotethatifVTVisnotpositivedenite,L(x;d)=L(x;d0)wheneverdd02N(V).Thuswemaycalldandd0inRNxequivalentifdd02N(V)andchoosethespaceofpossibleestimatestobethesetofequivalenceclassessoobtainedinsteadofRNx.Inthiscase,thespaceofpossibleestimatesisequippedwiththemetric(~d;~d0)=jVyV(dd0)j,where~d;~d02D,disanyelementin~d,andd0isanyelementin~d0.ThischoiceforDisequivalenttochoosingD=N(V)?withtheusualEuclideannormsinceforanyequivalenceclassinDthereisassociatedapointinN(V)?andviceversa.ItcanveriedthatwitheitheroneofthesechoicesforD,theresultsfor(;D;R)arestillvalid.Toshowthattheresultsfor(;L;R)arestillvalid,itisnecessary,todeneaclassL0=f(N(V)?A;N(V)?b):(A;b)2Lg.ItisobviousthatL0L.Certainlyforeach^x2L0,^x(y)isinthenewspaceofpossibleestimates.Now,itisstraightforwardtoshowthatalltheresultsofthischapterarestillvalidfor(;L0;R)andhencefor(;L;R). 48

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9.1 ,ifConditions 4.1 4.4 holdandiscompact,theproblemofndingaleastfavorableaprioridistributionisdualtotheproblemofndingaminimaxestimator.Thusintherestofthischapter,weconcerntheproblemofndingaleastfavorableaprioridistribution.Thefollowingtheoremisessentiallysimilartoatheoremin[ 18 ]andtheiterativealgorithmproposedin[ 38 ]. 7.1 and 7.2 hold.Constructasequencefig1i=12~Masfollows.Let1beanydistributionin~M.Havingchosen1;:::;i2~M,leti2besuchthatR(i;^xi)=sup2R(;^xi).Leti;=i+(1)i.Letibesuchthat~r(i;i)=sup2[0;1]~r(i;)andleti+1=i;i.Thenthesequencefigconvergesweaklytoaleastfavorableaprioridistribution. Proof. 18 ,Theorem2.3],withslightmodications. ThemaindifcultyinthealgorithmdescribedinTheorem 7.1 isinndingi2suchthatR(i;^xi)=sup2R(;^xi)fori1.Anotherdifcultyisduetothefactthat,ingeneral,sincethesupportofimaygrowasigrows,thecomplexityofthealgorithmcalculationsalsogrowswithi.Theproblemofndingisuchthat~r(i;i)=sup2[0;1]~r(i;)isaddressedbelowandcanbesolvednumerically.Insomespecialcases,itiseasytondi2suchthatR(i;^xi)=sup2R(;^xi)fori1andthecomplexityofthealgorithmcalculationsremain 49

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7.1 ispracticalinndingminimaxestimators.Onesuchcaseiswhenisaniteset.Inthesequel,weshowthatwhenweconsiderlinearestimationwithquadraticlossfunction,thealgorithmofTheorem 7.1 canbeoftenusedtondaminimaxestimator.Inthemoregeneralcase,thisalgorithmcanbeusedjusttond-minimaxestimators,whicharegoodapproximationstominimaxestimatorsforsufcientlysmall.Wediscussthederivationof-minimaxestimators,ingreatdetail,inthenextchapter.Theproblemofndingi2[0;1]suchthat~r(i;i)=sup2[0;1]~r(i;)isastandardoptimizationprobleminR:maximize~r(i+(1)i)subjectto2[0;1]:Fix1;22~M,andlet=1+(1)2for2[0;1].Intherestofthischapterweconsiderthefollowingoptimizationproblem,whichincludesthepreviousoneasaspecialcase: maximize~r()subjectto2[0;1]: Proof. LetD()=r(1;^x)r(2;^x)for2[0;1]. Proof. Asaconsequenceofthislemma,D(0)isthesubderivativeof7!~r()atthepoint02(0;1). 7.2 holds.Then7!D()ismonotonicallydecreasingandcontinuouson[0;1]and~r(0)=sup2[0;1]~r()for02[0;1]ifD(0)=0.

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7.2 furnishesthecontinuityof7!D()on[0;1].SinceD()isthesubderivativeoftheconvexfunction7!~r(),7!D()ismonotonicallydecreasingon(0;1).Bycontinuity,7!D()ismonotonicallydecreasingon[0;1].Fix02[0;1].SupposeD(0)=0.Thenbylemma 7.2 ,~r()~r(0)forall2[0;1].Itfollowsthat~r(0)=sup2[0;1]~r(). 7.2 and 7.3 implythatwecanuserelativelysimplenumericalalgorithmstosolve( 7 )numerically.Supposer(1;^x1)=r(2;^x1).Then~r(1)=r(;^x1)forall2[0;1].Itfollowsthat~r(1)~r()forall2[0;1]andhence=1isasolutionof( 7 ).Similarly,ifr(1;^x2)=r(2;^x2),=0isasolutionof( 7 ).Usingthefactthat7!D()iscontinuousandmonotonicallydecreasingon[0;1],andthefactthat2[0;1]isasolutionof( 7 )ifD()=0,theoptimizationproblem( 7 )canbesolvedasfollows:IfD(1)0,then=1isasolutionof( 7 ).IfD(0)0,then=0isasolutionof( 7 ).Finally,ifD(0)>0andD(1)<0,thereexists2(0;1)suchthatD()=0andhencesolves( 7 ).Inthiscase,sinceD()=0and7!D()ismonotonicallydecreasingon(0;1),canbeeasilyfoundusingthebisectionmethod.

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7.1 .Inpractice,itmaybenecessarytondan0-minimaxestimatorfor(f;X;R)suchthatthisestimatorisan-minimaxestimatorfor(;X;R).Inthelattercase,thealgorithminTheorem 7.1 canstillbeused,butitisnecessarytohaveaconditionthatenablesustocheckwhentherequiredprecisionisachieved.Checkingifanestimator^x,anessentiallyuniqueBayessolutionrelativeto,isan-minimaxestimatorcanbedoneusingthefollowinglemma: 7.1 holdsandsup2R(;^x)~r(),then^xisan-minimaxestimator. Proof. TheconditionofLemma 8.1 isonlyasufcientconditionforan-minimaxestimator.However,undercertainconditions,ifthenumericalalgorithmconvergestoaleastfavorableaprioridistribution,thenthereexistsanintegerNsuchthatLemma 8.1 issatisedfortheNthiteration. 7.1 and 7.2 hold.Letfig2Mbeasequencethatconvergesweaklytoaleastfavorableaprioridistribution02M.ThenthereexistsanintegerNsuchthatsup2R(;^xi)~r(i)foralliN.

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7.2 holds,itisstraightforwardthat SinceR(;^x0)iscontinuouson,itcanbeshown,asintheproofof[ 38 ,Theorem2part3],that ByTheorems 4.1 and 4.5 ,thereexistsaminimaxestimatorand(;X;R)isstrictlydetermined.ByTheorem 4.6 ,^x0isaminimaxestimator.Thus By( 8 )( 8 ),sup2R(;^xi)~r(i)!0andthelemmaisproved. Thefollowingconditionisneededforsomeoftheresultsofthischapter. 7.1 holds,andthefamilyfR(;^x):2Mgisequicontinuouson.Thenforany>0,thereexistsa>0suchthatforanynite-densesubsetfof,thefollowinghold: 1. Forany2M,thereexistsaprobabilitymeasure02fsuchthatR(;^x0)R(;^x)

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4.7 ,thatthereexistsaprobabilitymeasure02fsuchthatR(;^x0)R(;^x)forall2f.Sincefis=2-denseininthesenseof,R(;^x0)R(;^x)+forall2andpart1)isproved.2)ClearlytheresultsofChapter 4 canbeusedfortheestimationproblem(f;X;R).Thusforanynitef,thereexistsaleastfavorableaprioridistribution02fand^x0isanadmis-sibleminimaxestimatorfor(f;X;R).Itfollowsthatmax2fR(;^x0)max2fR(;^x)forany2M.Sincefis=2-denseininthesenseof,sup2R(;^x0)sup2R(;^x)+=2.Thus^x0isan=2-minimaxestimatorfor(;X;R). 8.1 part1)isan-completeclasstheorem,providedthatBisessentiallycomplete(thereaderisreferredto[ 39 ]forthedenitionofan-completeclassofdecisionfunctions).ItdiffersfromWolfowitz's-completeclasstheoremin[ 39 ]andhastheadvantagethateachelementinthisclassisadmissible.Theorem 8.1 part2)impliesthatanadmissible-minimaxestimatorcanbefoundinthefollowingway:Partitionthesettodisjointsets1,2;:::;Nsuchthatthediameterofi(i=1;:::;N)doesnotexceed.Ineachsetitakeapointi.Letf=f1;2;:::;Ng.Solveforaminimaxestimatorfor(f;X;R).Ifissufcientlysmall,thentheresultingestimatoris-minimaxfor(;X;R).Theorem 8.1 doesnotspecifyhowtochoosethatwouldguaranteeacertain.However,itisclearfromtheproofthatitissufcientthatsatisesjR(;^x)R(0;^x)j=2whenever2M,;02,and(;0).Infactforthischoiceof,itisenoughderivean=2-minimaxestimatorfor(f;X;R)andtheresultedestimatoris-minimaxfor(;X;R).

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8.1 thatthefamilyfR(;^x):2Mgisequicontinuousonisratherstrong.Whileitisoftensatisedinthecasethattheriskisbasedonalossfunctionthatisuniformlybounded,itmaynotbesatisedinthecasethatthelossfunctionisunbounded.Another,difcultywithTheorem 8.1 isthattherequirementfromthesetfisverystrong.AsetfthatisconstructedaccordingtothistheoremhasthepropertythatanyestimatorcanbeapproximatedbyaBayessolutionrelativetoameasurewithsupportinfwithdegradationofnomorethan.However,wearemainlyinterestedinapproximatingaminimaxestimatorandnotanyestimator.ThusitmaybepossibletochooseanitesetwhosecardinalityissignicantlysmallerthanfinTheorem 8.1 .Duetotheabove,wearenotgoingtouseTheorem 8.1 inthesequel,andwearegoingtoderivemethodsthatdonotrequireequicontinuity.ItisconvenienttousethenotionofFrechetdifferentiability.LetB(X;Y)denotethesetofboundedlinearoperatorsfromanormedlinearspaceXtoanormedlinearspaceY.Givenf2B(X;Y),jjfjjdenotestheoperatornormoff,i.e.,jjfjj=supx:jjxjj1jjfxjj. jjhjj=0:WecallAtheFrechetderivativeoffatxanddenoteitbyDf(x).Iff:X!YistwiceFrechetdifferentiableatx2X,weletD2f(x)denotethesecondFrechetderivativeoffatx.NotethatD2f(x)2B(X;B(X;Y)).Given^x2X,letDR^x()andD2R^x()denotetheFrechetderivativeandthesecondFrechetderivative,respectively,ofR(;^x)at.

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7.1 and 8.1 holdandiscompact.Supposeforany2~M,thereexistsanextensionofR(;^x)fromtoanopenconvexsetUCsuchthatR(;^x)isFrechetdifferentiableonUandthereexistsarealnumberMsuchthatjjDR^x()jjMsup2R(;^x)forall2~Mand2.Fix>0and0<0<,andlet=0 Proof. ThecardinalityofanitesubsetfofthatisconstructedaccordingtoLemma 8.3 maystillbeverylarge;thismaycausethecalculationofaleastfavorableaprioridistributionfor(f;X;R)tobeformidable.Inaspecialbutveryimportantcaseofcompact,itispossibletoderiveanite-densesubsetofsuchthatdependslinearlyonq 8.3

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36 ,Theorem3.20],thesetco(@)iscompactandhencetheKrein-Milmantheorem[ 36 ,Theorem3.23]impliesthat=co(@). Proof. LetSN=fa2RN:jjajj1=1g.

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8 )isfalse.Thenthereexists02RN+\SNandanonemptyindexsetJ06=Jsuchthat=PNi=10(i)i,0(i)>0foralli2J0,and0(i)=0foralli=2J0.Let0=(+0)=2.Then=PNi=10(i)i,0(i)>0foralli2J[J0,and0(i)=0foralli=2J[J0.SinceJ06=J,jJ[J0j>M,whichisacontradictiontothedenitionofM.Hence( 8 )musthold. 7.1 and 8.1 hold,isacompactconvexsubsetofthenormedspaceC,and@isaniteset.Supposeforeach2~M,thereexistsanextensionofR(;^x)fromtoanopenconvexsetUCsuchthatR(;^x)istwiceFrechetdifferentiableonUandthereexistsapositiverealnumberMsuchthatjjD2R^x()jjMsup2R(;^x)forall2~Mand2.Let>0and0<0<.Let=q Proof. 8.5 ,thereexistsanindexsetJf1;2;:::;Ngsuchthat( 8 )holdsfor.LetA=fi:i2Jg.Sinceisnotinf,isnotanextremepoint(Lemma 8.4 )andjJj>1.Let02f\E(co(A))besuchthat(;0)<.Then0=Pi2J(i)iforsome2RN+\SNsuchthat(i)=0fori=2J.Since=Pi2J(i)iforsome2RN+\SNsuchthat(i)>0

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2jj0jj2Csup2[0;1]jjD2R^x0(0+(1))jj:Bythehypothesisofthetheorem,jj0jj2Csup2[0;1]jjD2R^x0(0+(1)jj2Msup2R(;^x0):Thussup2R(;^x0)sup2fR(;^x0)R(;^x0)R(0;^x0)2M=2sup2R(;^x0).Itcanbeveriedthatsincesup2fR(;^x0)V(f;X;R)(1+0)andV(f;X;R)V(;X;R),thensup2R(;^x0)V(;X;R)(1+).Thus^x0isa-optimalestimator. SupposethenormedspaceCinCondition 8.1 isRNwithitsusualnorm.Inthiscase,ifisacompactconvexsetand@isnite,issaidtobeaconvexpolytopeinRN. 8.6 7.1 and 8.1 hold,andisaconvexpolytopeinRN.Supposeforeach2~M,thereexistsanextensionofR(;^x)fromtoanopenconvexsetUCsuchthatR(;^x)istwiceFrechet

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Proof. 8.6 ,let02Mbesuchthatthesupportof0iscontainedinf,x0isa(0;f)-optimalestimator,and2besuchthatsup2R(;^x)=R(;^x).Then2E(co(A))forsomeA22@.Let02f\E(co(A))besuchthatj(0)vij(i).BytheTaylortheorem,wehavethatforsome~2inthelinesegmentconnectingand0R(0;^x0)=R(;^x0)+1 2(0)TD2R^x0(~)(0):Certainlythereexistsa1;a2;:::;aN2Rsuchthat0=PNi=1aivi.Bythehypothesisofthetheorem,(0)TD2R^x0(~)(0)(0)TA0(0)sup2R(;^x0)=NXi=1a2iisup2R(;^x0):Sinceai=(0)vi,a2i(i)2.Thussup2R(;^x0)sup2fR(;^x0)R(;^x0)R(0;^x0)PNi=1(i)2i=2sup2R(;^x0).Itcanbeveriedthatsincesup2fR(;^x0)V(f;X;R)(1+0)andV(f;X;R)V(;X;R),thensup2R(;^x0)V(;X;R)(1+).Thus^x0isa-optimalestimator. 60

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9.1 ,appearsasaconjecturein[ 16 ].Weimposethefollowingadditionalconditionsinordertoderivetheresultsofthischapter: C0<+1.LetR(;^x)=r(;^x)+(1)R(;^x)andr(;^x)=RR(;^x)d,where01.LetK(^x;)=sup2R(;^x)andletK()=inf^x2XK(^x;).Sincesup2R(;^x)r(;^x),wehavethat 9.1 and 9.2 hold.ThenKisconcave,decreasing,andcontinuouson[0;1]. Proof. B LetG(^x)=sup2R(;^x)r(;^x). Proof. C 61

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Proof. 4.1 and 4.2 holdfor(;X;R),thereexistsaminimaxestimatorintheestimationproblem(;X;R).If,inaddition,iscompactandCondition 4.3 holds,thereexistsaleastfavorableaprioridistribution02M,andanyminimaxestimatorisaBayessolutionrelativeto0.AminimaxestimatorisanessentiallyuniqueminimaxestimatorandisadmissibleifCondition 4.4 holdsaswell.Infact,itcanbeshownthatif0<1,eachoneofConditions 4.1 4.4 holdsfor(;X;R)ifitholdsfor(;X;R). 7 fortheestimationproblem(;X;R),where0<1,providedthatConditions 7.1 7.2 holdfortheestimationproblem(;X;R).Infact,ifiscompact,itissufcientthatCondition 7.1 7.2 wouldholdfortheestimationproblem(;X;R).Indeed,supposeiscompactandConditions 7.1 7.2 holdfortheestimationproblem(;X;R).ItfollowseasilyfromLemma 9.3 thatforany2M,thereexistsanessentiallyuniqueBayessolutionrelativetointheestimationproblem(;X;R),whenceCondition 7.1 holds.Let^xdenotethe(essentiallyunique)Bayessolutionrelativetointheestimationproblem(;X;R).Letfig1i=1beasequenceinMthatconvergesweaklyto02M.Let0i=+(1)iforalli0.Thenf0igconvergesweaklyto00.SinceR(;^xi)=R(;^x0i)foralli0,R(;^xi)convergestoR(;^x0)uniformlyonthecompactset.Sincetheconvergenceisuniform,r(;^xi)convergestor(;^x0).ThusR(;^xi)convergestoR(;^x0)uniformlyoncompactsubsetsofandCondition 7.2 holds. 9.1 9.2 hold.LetQ()=+,whereG(^x)and2R.SupposeQ()>K()forall2[0;1].

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Proof. 9.1 ,Kisalsocontinuouson[0;1].Thusthereexists>0suchQ()K()>forall2[0;1].ItiseasytoverifythatfK(^x;):^x2X( Leti=i=nand^xibeaminimaxestimatorfor(;X;Ri)fori=0;1;:::;n.Notethat^xn=^xand^xiexistsforall0in1byTheorem 4.1 andRemark 9.1 .Since^x0isaminimaxestimatorfor(;X;R)and^xn=^x,G(^xn)G(^x0).ThusbyLemma 9.2 part1),thereexists1mnsuchthatG(^xm)G(^xm1).ByLemma 9.2 part3),^xm12X( Using( 9 ), By( 9 )and( 9 ),jK(m)~K(m)j<.Itfollows~K(m)

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9.1 9.2 hold,andV(;X;R)K(1).ThusQ()>K()forall01.Let=C0r(;^x0).ThenG(^x)andQ()=+C0.Let^xbeaminimaxestimatorfor(;X;R).Since^x0isarestrictedriskBayessolutionrelativeto(;C0)andC0>V(;X;R),r(;^x0)r(;^x).Thus>G(^x).ByLemma 9.4 ,thereexists^x02XsuchthatK(^x0;)K()forall01.Sincesup2R(;^x0)=C0,K(^x0;)=Q()andbyLemma 9.4 ,thereexists^x0suchthatsup2R(;^x0)

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9.1 9.2 hold.Letfng1i=1beasequencein[0;1]thatconvergesto02(0;1).Let^xibeaminimaxestimatorfor(;X;Ri)fori=0;1;:::.ThenifV(;X;R)0suchthatM>V(;X;R)andM+< C0.ByTheorem 4.2 ,thereexistrestrictedriskBayessolutionsrelativeto(;M)and(;M+).Therefore,Theorem 9.1 yields0and00in(0;1)suchthatsup2R(;^x0)=Mandsup2R(;^x00)=M+,where^xisaminimaxestimatorfor(;X;R)forall2(0;1).ByLemma 9.2 part3),0000.Infact,sincesup2R(;^x0)

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66

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Sinceforeach^x2X,sup2R(;^x)sup2R(;^x), By( 10 )and( 10 ),^xisaminimaxestimatorforx.Thuswiththeaboveriskfunction,minimaxestimationforxsubjectto(Sx;Sy;h)iscompletelydeterminedbyminimaxestimationforx(2Sx).Letusconsideranotherpossibilityfortheriskfunction.SupposeSxN.Letw:Sx![0+1).WecandenetheriskfunctionR(;^x)=X2SxR(;^x)w():

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1. IfConditions 4.1 and 4.2 holdfor(;X;R)foreach2Sx,thenConditions 4.1 and 4.2 holdfor(;X;R). 2. SupposeSxisnite.ThenifConditions 4.3 and 4.4 holdfor(;X;R)foreach2Sx,thenConditions 4.3 and 4.4 holdfor(;X;R). Proof. 4.1 holdsfor(;X;R).Foreach2SxletXbeacompactmetrizablespaceandRafunctionfromXinto[0;+1]suchthat(X;R)(X;R),R(;)islowersemicontinuousonXforeach2,andR(;a)2m(B())foreacha2X.LetX=Q2SxXbeequippedwiththeproducttopology.LetR(;^x)=P2SxR(;^x)w().ThenbyTychonoff'sTheorem,Xiscompact.Inaddition,Xismetrizablesinceitisacountableproductofmetrizablespaces.Sinceforeach2Sxand2,^x7!R(;^x)islowersemicontinuous,foreachnitesubsetSofSx,^x7!P2SR(;^x)w()islowersemicontinuous.Sincethepointwisesupremumofanycollectionoflowersemicontinuousfunctionisalowersemicontinuousfunction,wehavethatR(;)islowersemicontinuousonXforeach2.Sincethepointwiselimitofasequenceofmeasurablefunctionsisameasurablefunction,itiseasytoshowthatR(;^x)2m(B()).Itisalsoratherstraightforwardtoshowthat(X;R)(X;R).ThusCondition 4.2 holdsfor(;X;R).2)Theproofisratherstraightforwardandisomitted. 68

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10.1 impliesthatunderratherweakconditionson(;X;R)(2Sx),thereexistsaminimaxestimatorfor(;X;R)andarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;X;R).ThetheoremisespeciallyusefulforthecaseofniteSxsinceitspeciesthatifcertainconditionsholdsforeachoneoftheestimationproblems(;X;R)(2Sx),theyholdfor(;X;R)andhencetheresultsofthepreviouschaptersarevalidfor(;X;R).

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10.3 72

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10.3 .Forthesakeofclarity,letusrepeattheformulationofthisproblem.Weconsiderthefollowingdiscrete-timelinearstochasticsysteminstate-spaceform: wherexn2RNx(n0)isthestatevector,yn2RNyisthesystemoutput,vn2RNyisthemeasurementnoise,wn2RNxisthemodelnoise,andHnandFnarematricesinRNyNxandRNxNx,respectively.Weassumethatthereisgivenanestimationspace(Sx;Sy;h),whereSxN,Sy=N,andforeach2Sx,h()isanitesubsetofSx.WeassumethatfHngandfFngareknownsequences,thesequencesfvngandfwngareuncorrelated,vnandvmareuncorrelatedforn6=m,andwnandwmareuncorrelatedforn6=m.Inaddition,theinitialstatex0andvn(wn)areuncorrelatedforalln0.Weassumex0isaGaussianrandomvectorandfvng(fwng)isasequenceofidenticalGaussianrandomvectors.Weassumethatthemeanandcovarianceofx0andthecovariancesofvnandwnareunknown;itisonlyknownthateachoneofthesequantitiesbelongtoacertainset.Thespaceofstatesofnature,inthiscase,is122,where1istheclassofpossiblemeanvectorandautocorrelationmatrixpairsforx0,2istheclassofpossiblemeanvectorandautocorrelationmatrixpairsforvn(n=1;2;:::),and3istheclassofpossiblemeanvectorandautocorrelationmatrixpairsforwn(n=1;2;:::).AsmentionedinExample 10.3 ,thesetisasubsetofanitedimensionalspacenormedspaceC(seethedenitionofCinExample 10.3 ).For=123,wherei2ifori=1;2;3,weleti()andi()denotethemeanvectorandautocorrelationmatrixofi,respectively,fori=1;2;3.Ourmainassumptionregardingthespaceisthat2()>0foreach2.WeconsidertheriskfunctionR(;^x)=P2SxR(;^x)w()forthisproblem,wherew()>0foreach2Sx, 73

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24 ],wheresuchconditionsarederivedforthespecialcaseofuncertaintiesintheinitialstate.LetusshowthatConditions 4.1 4.2 ,and 4.4 holdfortheestimationproblems(;D;R)and(;L;R)foreach2Sx,andthatCondition 4.3 holdsaswellifiscompact.Fix2Sx.OurrststepistoshowthatConditions 5.1 5.7 hold.SincethelossfunctionLisquadratic,itisonlylefttoshowthatConditions 5.2 and 5.4 hold(Chapter 6 ).ConsidertheconditionaldistributionofYh()givenX=x,whenisthetruestateofnature.LetP(jX=x)denotethisconditionaldistribution,whichiscertainlyGaussian.Moreover,since2()>0foreach2,thefamilyfP(jX=x):2;x2XgisdominatedbytheLebesgue-Borelmeasureon(Yh();B(Yh())),whichisdenoted.LetP(jX=x)denotetheconditionaldistributionofYh()givenX=x,whenisthetruestateofnature.LetA2B(Yh())besuchthat(A)=0.ThenP(AjX=x)=0foreachx2X.ItcanbeveriedthatthisimpliesthatP(AjX=x)=0foreachx2X.ThusthefamilyfP(jX=x):2;x2XgisdominatedbyandCondition 5.2 holds.PutZ=[YTh()XT]T.Letfngbeasequenceinthatconvergesto02.Theni(n)!i(0)andi(n)!i(0),fori=1;2;3.ItcanbeveriedthatthisimpliesthatEn(Z)!E0(Z)andEn(ZZT)!E0(ZZT).ItfollowsthatPnconvergesweaklytoP0.Sincethesequencefngisarbitrary,fPngconvergesweakly 74

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40 ].ThusCondition 5.4 holds.Itisalsoclearthat7!E(Z)and7!E(ZZT)arecontinuouson.ItnowfollowsfromtheresultsofChapter 6 thatConditions 4.1 4.2 ,and 4.4 holdfor(;D;R)and(;L;R)foreach2Sx,andCondition 4.3 holdsaswellifiscompact.ByTheorem 10.1 ,whether(;D;R)or(;L;R)isconsidered,thereexistsaminimaxestimator,andthereexistsarestrictedriskBayessolutionrelativeto(;C0)foreach2MandC0V(;D;R).SupposeinadditionthatSxisniteandiscompact.Thenwehavethefollowingresultsfortheestimationproblem(;D;R):(;D;R)isstrictlydetermined,thereexistsaleastfavorableaprioridistribution02Mandaconditionalmeanestimatorrelativeto0isanessentiallyuniqueadmissibleminimaxestimator.Moreover,theclassofconditionalmeanestimatorsisessentiallycomplete.Notethat,ingeneral,aconditionalmeanestimatorrelativetoisnotaLMMSEestimatorsincemaynotassignmass1toasinglepointin.Similarly,wehavethefollowingresultsfortheestimationproblem(;L;R):(;L;R)isstrictlydetermined,thereexistsaleastfavorableaprioridistribution02MandaLMMSEestimatorrelativeto0isanessentiallyuniqueadmissibleminimaxestimator.Considerthelteringproblem.ThenaLMMSEestimatorrelativetoisnotnecessarilyaKFifdoesnotassignmass1toasinglepointin.ThereisaspecialandimportantcaseinwhichaLMMSEestimatorwithrespectto2MisaKF.Wewilltreatthiscaseinthesequel.Inthefollowingchaptersweconsidertwospecialcasesoftheaboveproblem.Therstcaseisthecaseofstochasticuncertaintiesintheinitialstate,modelnoise,andobservationnoisewithLastheclassofavailableestimators.ThesecondcaseisthecaseofdeterministicuncertaintiesintheinitialstatewithDastheclassofavailableestimators.InbothcaseswewillassumethatSxisniteandiscompact.NotethatbyRemark 4.1 ,thecasethatisbounded,butnotnecessarilycompactisalsocovered.Thesetwocasesareimportantontheirownmerit,andtheywillalsobeusedtoillustratesomeofthegeneralresultsofpreviouschapters.Forexample,therstcasewillbeusedtoillustratethemethodproposedinChapter 9 toderivearestrictedriskBayessolution 75

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9.3 )andthesecondcasewillbeusedtoillustratethemethodproposedinChapter 8 toderiveanapproximationforminimaxestimators. 76

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11 .Weconsiderthecaseofstateestimationwithstochasticuncertaintiesintheinitialstate,modelnoise,andobservationnoisewiththeclassofavailableestimatorsbeingtheclassofafneestimators.Throughoutthischapter,weassumeiscompact.ThusV(;L;R)<+1.Inthischapter,weassumethattheestimationspace(Sx;Sy;h)issuchthatSx=f0;1;:::;ntg,Sy=N,andh(k)=f0;1;:::;nkgforeachk2Sx,wherentandnk,fork=0;1;:::;nt,arenonnegativeintegers.Weassumethatw(k)=1foreachk2Sx,i.e.,R(;^x)=Pntk=0Rk(;^xk).GivenC0V(;L;R)and2,ourgoalistondarestrictedriskBayessolutionrelativeto(;C0). wherewn0=[wT0wT1wTn]Tandvn0=[vT0vT1vTn]T.Letk0andn0.By( 12 )and( 12 ),Ayn0+bxk=(AOnFk1;0)x0+n_k1Xi=0(ATi+1nFk1;i+1)wi+Avn0+b:

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Let()=[1()T2()T3()T]Tandlet()betheblockdiagonalmatrixwith1(),2(),and3()initsdiagonalblocks.Let;i()=i()i()i()Tfori=1;2;3.Weassumethatsomeaprioriinformationregardingthetruestateofnature2isavailable.Thecasethatnoaprioriinformationisavailableisanimportantspecialcase.Theaprioriinformationisgivenintheformofanominal2.Forsimplicity,weassume1()=0.Thereisnolossofgeneralityinthisassumptionsinceif1()6=0,wecantranslatethestatesandobservationsandbringtheproblemtothisform.Below,wesummarizetheassumptionstakensofartogetherwithsomenewassumptions. 78

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12.1 12.4 holdinthesequel.NotethatisthenacompactconvexsubsetofC.By( 12 ),Rk(;^xk)isbothconvexandconcaveonforall^xk2Lk,i.e.,forany0<<1and0;002,Rk(0+(1)00;^xk)=Rk(0;^xk)+(1)Rk(00;^xk):Inaddition,Rk(;^xk)iscontinuousonforall^xk2Lk.ItfollowsthatR(;^x)isbothconvexandconcaveonandcontinuousonforall^x2L.Letr(;^x)=RR(;^x)dandletrk(;^xk)=RRk(;^xk)dfork=0;1;:::;nt.LetZbetheclassofallnitesubsetsof.Let~MdenotethespaceofdistributionsinMwithnitesupport.Let2~M.ThenthereexistsZ2Zand1;:::;jZj2suchthatZ=f1;:::;jZjgand(Z)=1.Inthiscase,let(i)denotethemassthatassignstothepointifori=1;:::;jZj.Finally,let=PjZji=1i(i).Sinceisconvex,2.SinceR(;^x)isbothconvexandconcaveonforall^x2L,r(;^x)=R(;^x)forall^x2L. Proof. 27 ,Appendix3].Letfigbeasequenceofdistributionsin~Mthatconvergesweaklyto.SinceR(;^x)iscontinuousonthecompactset,R(;^x)isboundedand 79

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SinceR(;^x)isbothconvexandconcaveonandihasnitesupport, By( 12 )( 12 ),r(;^x)=R(;^x).Theprooffollowsfromthearbitrarinessof^x. ItisclearfromtheproofofLemma 12.1 thatif2~M,^xisaBayessolutionrelativetoifandonlyif^xisaBayessolutionrelativeto,where=.WewanttoapplytheresultsofChapter 9 fortheestimationproblem(;L;R).WehavealreadyshownthatConditions 4.1 4.4 holdinChapter 11 .LetusshowthatConditions 7.1 7.2 ,and 9.1 holdaswell.InChapter 11 itwasshownthatLhastheproperty(W)andCondition 4.4 holds.ThusTheorem 4.2 impliesthatCondition 7.1 holds.CertainlytheweakerCondition 9.1 mustholdaswell.ItislefttoprovethatCondition 7.2 holdsfor(;L;R).Infact,itissufcienttoprovethatCondition 7.2 holdsfor(;Lk;Rk)foreachk2Sx.Fixk2Sx.Forthesakeofclarity,weprovethatCondition 7.2 holdsfor(;Lk;Rk)inthecasethat1()=0forall2.ItiseasytoverifythatthisistruealsointhemoregeneralcaseofAssumption 12.2 buttheexpressionsarerathercumbersome.Assuming1()=0forall2andusing( 12 ), 80

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whereweextendk;ifromto~Mbydeningk;i()=k;i()forall2~M(i=1;2).LetfigbeasequenceinMthatconvergesweaklyto02M.ByLemma 12.1 ,thereexistsasequencefig2andanelement02suchthatRk(;^xi;k)=Rk(;^xi;k)(i=0;1;:::).Certainlyk;iiscontinuousonfori=1;2.Itfollowseasilyfrom( 12 )thatthemapping7!^x;kiscontinuousonthecompactsetandhenceuniformlycontinuousandbounded.Thusthereexists>0suchthat^x;k2Bforall2,whereB=f^xk2Lk:jj^xkjjLkg.ItiseasytoseethatRkiscontinuousonthecompactsetB.ThusfRk(;):2gisequicontinuousonB.Since7!^x;kisuniformlycontinuouson,thefamilyf7!Rk(0;^x;k):02gisequicontinuousonandthereforeRk(;^xi;k)convergestoRk(;^x0;k)uniformlyon.ItfollowsthatRk(;^xi;k)convergestoRk(;^x0;k)uniformlyonandCondition 7.2 holds.Let^xdenotethe(essentiallyunique)Bayessolutionrelativeto2Mintheestimationproblem(;L;R).SinceiscompactandR(;^x)iscontinuouson, 9.2 holdsifV(;L;R)< C0.Let^x=^x+(1).ByLemma 9.3 ,^xisaBayessolutionrelativetointheestimationproblem(;L;R).Letr(;^x)=r(;^x)+(1

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4.2 4.4 7.1 ,and 7.2 holdfortheestimationproblem(;L;R),theyholdfortheestimationproblem(;L;R)for0<1(Remarks 9.1 and 9.2 ).ThususingtheresultsofthepreviouschaptersandLemma 12.1 ,if0<1,thereexists02suchthat~r(0)=sup2~r()and^x0isanadmissible,essentiallyuniqueminimaxestimator.Inparticular,bysetting=0,thereexistsaminimaxestimatorfor(;L;R).Letusconsidertheproblemofndingaminimaxestimatorfortheestimationproblem(;L;R),where2[0;1).Asmentionedearlier,ifV(;L;R)

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Proof. 2(+0).Then20.ByLemma 7.1 ,~r()~r().Thusthereexistsaleastfavorableaprioridistribution0thatisin0.Certainly0isalsoaleastfavorableaprioridistributionintheestimationproblem(0;L;R).Since0<1,R(;^x)=sup2R(;^x)ifandonlyifR(;^x)=sup2R(;^x).Since0isacompactsubsetof,wemayapplyTheorem 7.1 fortheestimationproblem(0;L;R)andderivetheabovealgorithm.NotethatthealgorithmissimpliedwiththehelpofLemma 12.1 sinceweneedtoconsideronlydistributionswithsupportofasinglepoint,i.e.,elementsof.Sincewehaveconsideredtheestimationproblem(0;L;R),thesequencefig1i=1isin0.ByTheorem 7.1 ,thesequenceconvergesweaklytoaleastfavorableaprioridistribution020.Thus^x0isaminimaxestimator.Since(i)=0and()=0,^xi2L0foralli0.SinceCondition 7.2 holds,R(;^xi)convergesuniformlyontoR(;^x0). ThemainstepsofthealgorithmofLemma 12.2 areSteps2and4.InStep2,weneedtosolvetheproblemofndingamaximizerofR(;^x)over0.WewilladdressthisprobleminSection 12.2 .Step4canbedoneusingnumericalmethodsasdescribedinRemark 7.1 .SupposeV(;L;R)

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12.2 ,weaddressthisproblemandareabletosolveitforsomeimportantcasesof. 12.2 andinthemethodtondarestrictedriskBayessolution,whichisdiscussedinRemark 9.3 .Weconsiderspeciccasesoftheparameterset0.Thereisoneimmediatecaseinwhichthisproblemhasasimplesolution.Thedenitionofanextremepointisneeded(Denition 21 ).RecallthatgivenasetA,weuse@AtodenotetheextremalboundaryofA,whichisthesetofallextremepointsofA.Since0isconvexandcompactandR(;^x)isconvexandcontinuouson,byBauer'sminimumprinciple[ 41 ],sup20R(;^x)=sup2@0R(;^x):If@0isnite,sup20R(;^x)=max2@0R(;^x).Thussup20R(;^x)canbeeasilycalculated.Fix^x2L0.By( 12 ),R(;^x)=P3i=1tr(i()i)forsome12SNx+,22SNy+,and32SNx+.Leti=fi():2gfori=1;2;3.Let=f(1();2();3()):2g.Itfollowsfromthedenitionofthat=123.Thussup20R(;^x)=sup(1;2;3)23Xi=1tr(ii)=3Xi=1supi2itr(ii):Therefore,weareleftwiththefollowingoptimizationproblem:GivenN>0,amatrix2SN+,andaconvexcompactsubsetAofSN+, maximizetr(A)subjecttoA2A: ConsidertheimportantcaseinwhichA=fA2SN+:f1(A)0;:::;fN(A)0g,wheref1;:::;fNareconvex(real-valued)functionssuchthatAiscompactandconvex.Then( 12 )is 84

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42 ,pp.167].Thusinmanyimportantcases,( 12 )canbesolvednumerically.Intherestofthissection,weconsiderseveralcasesinwhich( 12 )hasananalyticalsolution.LetA02SN+andD2RNNbeanonsingularmatrix.Let~=(D1)TD1and%1%2:::%Ndenotetheeigenvaluesof~.LetB2SNand12NbethesingularvaluesofB.ThenbyatraceinequalityofvonNeumann[ 43 ], WeconsiderthefollowingpossibilitiesforA:1)A=fA2SN+:tr(DADT)1g.FixA2A.Let12NdenotetheeigenvaluesofDADT.By( 12 ),tr(A)=tr(DADT~)PNj=1j%j.Thustr(A)tr(DADT)%1.SinceA2A,tr(A)%1.LetA=D1~u~uT(D1)T,where~uistheeigenvectorof~correspondingto%1.ThenA2Aandtr(A)=%1,whenceAmaximizestr(A)overA.2)A=fA2SN+:jjD(AA0)DTjjF1g,wherejjjjFistheFrobeniusnorm.Let12NdenotethesingularvaluesofD(AA0)DT.By( 12 )andCauchy-Schwarzinequality,tr((AA0))PNj=1j%jq PNj=12jq PNj=1%2j.Thustr(A)jjD(AA0)DTjjFjj~jjF+tr(A0).SinceA2A,tr(A)jj~jjF+tr(A0).SupposeA2Aandtr(A)=jj~jjF+tr(A0).ThentrD(AA0)DT~

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12 ),tr((AA0))1PNj=1%j.Thustr(A)tr(~)jjD(AA0)DTjj2+tr(A0).SinceA2A,tr(A)tr(~)+tr(A0).LetA=A0+D1(D1)T.ThenA2Aandtr(A)=tr(~)+tr(A0),whenceAmaximizestr(A)overA.4)A=fA0+(1)A:jjDADTjjF1g,where0<<1.CertainlyA=A0+(1)D1~(D1)T

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PjD1uifori=1;:::;Nx.Since,~uTiDTD~ui=Pj1,~ui2X0and~ui2X0fori=1;:::;Nx.Letbethedistributionthatassignsmass~i=2to~uiand~i=2to~ui(i=1;:::;Nx).Then2MX0,haszeromean,andA=RX0x0xT0d.ThusA21Itfollowsthat1=fA2SNx+:tr(DADT)1g.Thiscasewasalreadytreatedinthissection.SupposeX0isaconvexpolytopethatissymmetricaroundthepoint0.Thenthereex-istanitenumberofpointsx0;1;x0;2;:::;x0;N2X0suchthatx0;iandx0;iareextremepointsofX0fori=1;2;:::;N.Certainlysup21tr(1)=sup2X0RX0tr(x0xT01)dsupx02X0tr(x0xT01).ByBauer'sminimumprinciple[ 41 ],supx02X0tr(x0xT01)=supx02@X0tr(x0xT01).SinceX0hasanitenumberofextremepoints,sup21tr(1)max1iNtr(x0;ixT0;i1).Sincex0;ixT0;i21fori=1;:::;N,sup21tr(1)=max1iNtr(x0;ixT0;i1)andthereexists1iNsuchthatx0;ixT0;imaximizestr(1)over1. 87

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wheren()=(;^xn()),njn1()=(;^xnjn1()),andKn()denotestheKalmangain.Considerthelteringproblemspeciedbytheestimationspace(Sx;Sy;h),whereSx=f0;:::;ntg,Sy=N,andh(i)=f0;:::;igforeachi2Sx.Then^x=(^x0();^x1();:::;^xnt())andisreferredtoastheKFrelativeto.Since2()>0,theexistenceoftheKFisguaranteed[ 44 ].ItiseasytoverifythatRk(;^xk(0))=tr(Wk(;^xk(0))).Usingtheresultsoftheprevioussections,thereexistsa2suchthattheKFrelativetoisarestrictedriskBayessolutionrelativeto(;C0)forallC0V(;L;R).Infact,thereisanotherinterestingpropertyregardingtheKF.UsingCorollary 9.1 andLemma 12.1 ,wehavethattheclassofKFsrelativeto2isessentiallycomplete.Thusaslongastheperformanceisjudgedsolelybasedontheriskfunction,ifthechoiceofestimatorsisrestrictedtoafneestimators,thennomatterwhatoptimalitycriterionisused,onemayconsideronlytheclassofKFsrelativeto2.OurnextstepistoderivemoreconvenientexpressionforRk(;^xk(0))inthecasethatand0arein0.Letk(;0)=(;^xk(0)).Using( 12 )( 12 ),itcanbeshownthat0j1(;0)=1()n(;0)=Kn(0)2()Kn(0)T+[IKn(0)Hn]njn1(;0)[IKn(0)Hn]Tn+1jn(;0)=3()+Fnn(;0)FTn:

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12 ).Recallthatifand0arein0,thenR(;^x0)=P3i=1tr(i()i(0)).Ourgoalistondanexpressionfori(0),fori=1;2;3,sinceitisneededinordertondamaximizerofR(;^x0)over0.Let~Fn(0)=FnFnKn(0)Hn,let~Fi;j(0)=~Fi(0)~Fi1(0)~Fj(0)(i>j)and~Fi;i(0)=~Fi(0).Wewillusetheconvention~Fi1;i(0)=I.LetCn;m(0)=(IKn(0)Hn)~Fn1;m(0)(0mn;0n),letDn;m(0)=Cn;m+1(0)FmKm(0)(n>m0)andDn;n(0)=Kn(0)(n0).NotethatCn+1;m=(IKn+1Hn+1)FnCn;mandCn;m1=Cn;m~Fm1.Thenn(;0)=Cn;0(0)1()Cn;0(0)T+nXm=0Dn;m(0)2()Dn;m(0)T+nXm=1Cn;m(0)3()Cn;m(0)T:Leti(0)=Pntn=iCn;i(0)TWnCn;i(0),fori=0;1;:::;nt.Theni=~FTii+1~Fi+(IKiHi)TWi(IKiHi):Itfollowsthat1(0)=ntXn=0Cn;0(0)TWnCn;0(0)=0(0);2(0)=ntXn=0nXm=0Dn;m(0)TWnDn;m(0)=ntXm=1Km1(0)TFTm1m(0)Fm1Km1(0)+ntXm=0Km(0)TWmKm(0)and;3(0)=ntXn=1nXm=1Cn;m(0)TWnCn;m(0)=ntXm=1m(0):Ingeneral,thecalculationof2(0)and3(0)requiresthestorageofK0(0);:::;Knt(0),whichmaybeproblematicforlargent.Thisisduetothefactthaticanbeupdatedbasedoni+1butnotviceversa.Nevertheless,intheimportantcasethatFiisinvertiblefor0int1,thecalculationof1,2,and3maybedoneinsuchawaythatthereisonlyaneedofaxedstorageplacethatdoesnotdependonnt.First,letusshowthatsince2(0)isinvertible, 89

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12 ),IKn(0)Hn=Injn1(0)HTn[2(0)+Hnnjn1(0)HTn]1Hn:Thusbythematrixinversionlemma,IKn(0)Hnisinvertibleand(IKn(0)Hn)1=I+njn1(0)HTn2(0)1Hn:SinceFiisinvertible,i+1=[(IKiHi)1F1i]Ti[IKiHi]1F1i(F1i)TWiF1i.Inthiscase,i+1canbeupdatedbasedoniandonlyaxedstorageplaceisneeded.WenowdiscusstheconnectionbetweentherestrictedriskBayesapproachandthe-minimaxapproachandillustratethatthe-minimaxapproachcanberegardedasaspecialcaseoftherestrictedriskBayesapproach.Theclassofaprioridistributionsinthe-minimaxapproachcoincideswiththeclassofthestatesofnatureintherestrictedriskBayesapproach.BysettingC0=V(;L;R),wehavethatforany2M,arestrictedriskaBayessolutionrelativeto(;C0)isaminimaxestimatorfor(;L;R).Since=,theriskR(;^x)is,infact,theBayesriskrelativetoacertaindistributioninifweadoptthe-minimaxformulation.Thusaminimaxestimatorfor(;L;R)isa-minimaxestimator.Therefore,theresultsofthisworkcanbeusedtonda-minimaxestimator.Wenotethatifsomeaprioriinformationisavailable,therestrictedriskBayesapproachispreferabletothe-minimaxapproachsinceitutilizesthisinformation.However,ifnoaprioriinformationisavailable,themostreasonablechoiceforC0seemstobeV(;L;R)andhenceweareleftwith-minimaxestimation. 45 ].Letxn2R3,wherexn(1)denotesthepositionofthetarget,xn(2)denotesthevelocityofthetarget,andxn(3)denotestheaccelerationofthetarget.Wealsoassumearadarmeasuresthepositionofthetarget.Hencethestatespacemodelin( 11 )is 90

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12.1 ,1=fA2S2+:tr(D1ADT1)1g.Inaddition,itisclearthat2=f2S1+:jjD2(0)DT2jj21g,and3=fQ2S2+:jjD3(QQ0)DT3jjF1g,whereD2=r1=2andD3=r1=2Q~D.Inthisexample,weconsiderthelteringproblemspeciedbytheestimationspace(Sx;Sy;h),whereSx=f0;:::;ntg,Sy=N,andh(i)=f0;:::;igforeachi2Sx.Wechoosethefollowingvalues:=0:01,nt=120,0=1104,q0=100,r=1000,rQ=30,D1=26666451060000:0020000:01377775;

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12.1 holdsinthisexample.WehaveshownthatAssumption 12.2 holdsinExample 12.1 .ItcanbeeasilyveriedthatAssumptions 12.3 and 12.4 holdaswell.OurrststepistochooseC0.InordertochooseC0,weneedtocalculateV(;L;R)and 12.2 tocalculate^xfor2[0;1)andthencalculater(;^x)andsup2R(;^x).Fig. 12-1 showstheplotoftheBayesriskr(;^x)versusthemaximumrisksup2R(;^x).NotethatbyTheorem 9.1 ,thisgure,infact,showstheBayesriskachievedbyarestrictedriskBayessolutionrelativeto(;C0)versusC0.HencethisguretellsusthetradeoffbetweenthepenaltyontheBayesriskandthesafeguardonthemaximumriskbyemployingrestrictedriskBayesestimation.Inthisexample,V=1:225105and 92

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9.3 andthealgorithmofLemma 12.2 .ItturnsoutthattherestrictedriskBayessolutionrelativeto(;C0)isaminimaxestimatorfortheestimationproblem(;L;R0),where0=0:986.WiththischoiceofC0,therestrictedriskBayesapproachcanreducethemaximumrisktoabout1/13thofthatofaBayessolutionwhilesufferingonlyminimally(about4%)ontheBayesrisk.Fig. 12-1 illustratesthebehavioroftheBayes,minimax,andrestrictedriskBayessolutionsintwoextremecases:Thecasethatisthetruestateofnatureandthecasethatthetruestateofnatureistheworstcasechoiceforeachoneoftheseestimators,respectively.However,Fig. 12-1 doesnotillustratethebehavioroftheseestimatorsforothervaluesofthetruestateofnature.Itisimportanttoevaluatetheperformanceoftheseestimatorsrelativetoall2thatarelikelytobethetruestateofnaturebasedonouraprioriinformation.Inordertodothat,weneedtomakesomeassumptionsregardingthesetof2thatarelikelytobethetruestateofnature.Weassumethatthissetis=f0+(10):01;2g,where0<<1,i.e.,thetruestateofnatureislikelytobean0-mixturebetweenandanunknown2for01.Themoreaprioriinformationwehave,themorelikelyitisthatthetruestateofnatureisclosetoandhencethelargeris.Notethatforany^x2L,sup2R(;^x)=r(;^x)+(1)sup2R(;^x).Let^xdenotethe(essentiallyunique)minimaxestimatorandrecallthat^x0denotesthe(essentiallyunique)restrictedriskBayessolutionrelativeto(;C0).Itiseasytoverifythatsup2R(;^x)=~r()+(1) 12-2 showsthemaximumriskoveroftheBayes,minimax,andrestrictedriskBayessolutionsfor2[0:95;1].Thisgureshowstheworstcaseperformanceoftheseestimatorsrelativetovalues 93

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AchievedBayesriskvs.maximumrisk 94

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ThemaximumriskoveroftheBayes,minimax,andrestrictedriskBayessolutionsvs.

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11 .Weconsiderthecaseofdeterministicuncertaintiesintheinitialstate.Weassumethatthesequencesfvngandfwng,whichhavezeromean,satisfyforn;m0 whereEdenotestheexpectationoperator.Thismeansthat2and3containonlyoneelement.Werestrictourselvestothecase2>0,whichisusuallythecaseinwellmodeledproblems[ 44 ].Itislefttospecifytheinitialstatevectorx0.Wemodelx0asadeterministic(unknown)parameterbelongingtoaparametersetX0.WeassumeX0isacompactsubsetofRNx.NotethatthecurrentformulationdeviatesfromthestandardKFassumptionsinthattheinitialstateisnotmodeledasrandomwithknownstatistics.Sinceweassumedeterministicuncertaintyintheinitialstatevector,1isthecollectionf(x0;x0xT0):x02X0gofmeanandsecondmomentpairs.NotethatsinceX0iscompact,iscompact.Weareinterestedthelteringproblemthatisspeciedbytheestimationspace(Sx;Sy;h),whereSx=f0;:::;ntg,Sy=N,andh(i)=f0;:::;igforeachi2Sx.Weletw(i)=1foreachi2Sx,i.e.,theriskfunctionissimplygivenbyR(;^x)=ntXn=0Rn(;^xn):Sincethereisaone-to-onecorrespondencebetweenandX0,weslightlyabusethenotationandassumethatthespaceofstatesofnatureisX0,i.e.,inthischapter=X0.WealsouseR(x0;^x)insteadofR(;^x),Rn(x0;^x)insteadofRn(;^x),forn=0;:::;nt,andetc. 96

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44 ]byinitializingtheKFwithzeromeanandcovariance.Itisstraightforwardtoverifythatn(x0)=0nforallx02X0.Let0n+1jn=3+Fn0nFTn,K0n=0njn1HTn[2+Hn0njn1HTn]1forn>0,andK00=0.Let~Fn=FnFnK0nHn,let~Fi;j=~Fi~Fi1~Fj(i>j)and~Fi;i=~Fi.Wewillusetheconvention~Fi1;i=I.TheproblemofstateestimationwitharandominitialstatewhosedistributioninnotnecessarilyGaussianisconsideredbyLainiotisetal[ 46 ]andtheCMestimatorforthisproblemisderived.Let^x0;n()denotetheCMestimatorwithrespecttoforx0basedonyn0,i.e.,^x0;n()=RX0x0pnx0(yn0)d 97

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46 ],itisnotdifculttogetthefollowingsetofequationsforn0: 2xT0Unx02xT0tn(yn0)+gn(yn0); wherethetermgnisafunctionofyn0anditsexactcalculationisunnecessaryforourpurpose.Thus^xn(),theCMestimatorwithrespecttoforxn,isthesumoftwoterms;onetermis^x0n,theCMestimatorforxnwhenthesystemhaszeroinitialconditions,andthesecondtermiscompletelyspeciedby^x0;n(),theCMestimatorwithrespecttoforx0.Analternativeinterpretationistoregard^xn()asthesumoftwoterms,wherethersttermisanestimatorofxnforknownzeroinitialconditionandthesecondtermaccountsfortheeffectoftheunknowninitialstate.AconditionalmeanestimatorwithrespecttoisanessentiallyuniqueBayessolutionrelativeto.Theexpressionfortheconditionalmeanestimatorisimportantsinceanessentiallyuniqueminimaxestimatoristheconditionalmeanestimatorwithrespecttoaleastfavorableaprioridistribution.Inaddition,theclassofconditionalmeanestimatorswithrespectto2MX0isessentiallycomplete.LetDL=f^x():2MX0g. 98

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8 .Ourgoalisthentoderivesuboptimalestimatorsthatcangivemaximumriskarbitrarilyclosetothatofaminimaxestimator.ThefollowingLemmasarederivedin[ 24 ]. BecauseofLemmas 13.1 and 13.2 andthefactthatX0isasubsetofnitedimensionalnormedspace,wecanusetheresultsofChapter 8 tonda-optimalestimator.InparticularifX0isaconvexpolytope,Lemma 8.7 canbeusedtonda-optimalestimator.Inordertonda-optimalestimator,weneedtoseta0<0

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7.1 ,intermsofthecomputationburden,sinceitmayrequirelesscalculationsoftheriskfunction.Thecomputationalcomplexityofa-optimalestimatorderivedaccordingtoLemma 8.7 dependslinearlyonthesupportof0.Itmayberatherhigh,anditiscertainlyhigherthanthecomplexityofafneestimators.InChapter 12 wediscussedtheproblemofrestrictedriskBayesestimationwhentheclassofestimatorsisrestrictedtoafneestimators.Whentheclassofestimatorsisrestrictedtoafneestimators,theminimaxproblemofthissectioncanberegardedasaspecialcaseoftheproblemconsideredinChapter 12 .Ifwecanndanafneestimatorthatisalsoa-optimalestimator,wewouldprobablyprefertheafneestimator.Ifsuchanafneestimatorexists,theresultsofthissectionarestillimportantsincetheygiveustightlowerandupperboundsonthemaximumriskandhenceenableustoevaluatetheperformanceoftheafneestimatorwiththebestpossibleperformanceinthesenseofthemaximumrisk.Wefurtherillustratethisinthefollowingnumericalexample. whereF=264101375andwniszeromeanGaussianrandomvectorwithcovariancematrix3=264000q375:

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whereH=[10]andvnisazero-meanGaussianrandomvariablewithvariance2.NotethatthisexampleisasimpliedversionofmanyrealapplicationsinwhichtheKFisused[ 21 ].Inthisexample,weassumethatX0=[1000;1000][50;50],=0:01,2=2500,q=25,nt=10,Wn=0forn<5,andWn=2641000375forn=5;:::;nt,whereWn=VTnVn.Ourgoalistoderivea-optimalestimatorfor=0:03.Thismeansthatthedegradation,whenusingthederivedestimatorinsteadofanexactminimaxestimator,isatmost3%ofthemaximumrisk.Theadmissible-optimalestimatorthatwederiveisaCMestimatorwithrespecttoadiscreteaprioridistribution.LetVbeamatrixsuchthattheithcolumnofVistheitheigenvectorof2U10.Letidenotetheitheigenvalueof2U10.Let=0:03and0=0:0023.Theabovechoicesof0resultsfromacertaintradeoff.Thechoiceof0determineshowdensethenitesubsetXf0ofX0andspecieshowclosetheresultedestimatoristoaminimaxestimatorfor(Xf0;D;R).Ifwechoose0veryclosetozero,wewoulddecreasejXf0j,butwewouldneedmoreiterationstonda(0;Xf0).Ifwechoose0toolarge,thedimensionofXf0wouldgrowandhencealsothecomputationalcomplexity.Let(i)=q 13-1 and 13-2 .Duetotheeigen-structureofthematrixU10,theresultedsetXf0isnotastandardgrid.ThepointsinthesetXf0aremuchdenserinthedirectionofthersteigenvectorthaninthedirectionofthesecondeigenvector.Next,wesolveforan(0;Xf0)-optimalestimator.WesettheinitialaprioridistributiontobetheuniformdistributiononXf0.Wethenupdatetheaprioridistributionusingavariantofthe 101

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24 ].Asmentionedearlier,itispossibletousealsothealgorithmofTheorem 7.1 .EachupdateincreasestheBayesrisk.Westopthealgorithmwhentherequiredtoleranceof0isachieved.Sincea(0;Xf0)-optimalestimatorisan-minimaxestimatorfor(Xf0;D;R)with=0V(Xf0;D;R),itispossibletocheckwhethertherequiredtoleranceisachievedusingLemma 8.1 .Let0denotetheresultedaprioridistribution;theresultedestimatoristhen^x(0),whichisa-optimalestimator.Fig. 13-3 showstheriskR(x0;^x(0))forx02Xf0.ThemaximumriskonthesetXf0is1864:6.DuetotheconstructionofXf0,themaximumriskonX0islessthan1916:1.InFig. 13-4 weshowtheresultedaprioridistribution0.Itseemsunreasonabletocomparebetweentheproposed-optimalestimatorwithanestimatorthatisderivedusingthe-minimaxapproach.Thereasonisthatbyassumingthatx0isdeterministicandbelongstoX0,thereisnosystematicwayastohowtochooseaclassofdistributions.Ofcourse,onecanchoosetobeMX0,butthena-minimaxestimatorcanbeshowntobeaminimaxestimator(i.e.,thetwoproblemsareequivalent).ItmaybethecasethataproperlyinitializedKFcanhaveaverycloseperformancetothatofaminimaxestimator.Ifoneadoptstheformulationofthiswork,thenarigorouswaytoinitializetheKFistoderivealinear(orafne)minimaxestimator,whichisaKFwithrespecttoaleastfavorableaprioridistribution.Inpractice,theKFisofteninitializedwithzeromeanandcovariance2I,where2ischosenheuristically.Wewishtoillustratethatthisheuristicmethodcansometimesleadtoundesirableperformance.InFig. 13-5 ,themaximumriskoftheKFinitializedwithzeromeanandcovariance2Iisplottedasafunctionof2[150;450].Itisapparentthatchoosingtoosmallleadstoaverypoorperformance.Inaddition,choosingtoolargemayalsoleadtoundesirableperformance.Thebestchoiceofintermsofthemaximumriskis=289:7andthemaximumriskinthiscaseis2042:8.Thusforthebestchoiceof,wehaveadegradationofatleast6:5%andatmost10%intermsofthemaximumriskrelativetotheestimator^x(0).Thisisnotasignicantdegradationandwhencomplexityistakenintoaccount,itmaybepreferabletouseaKFthatisinitializedwith=289:7.Inthisexample,itisclearthatthedegradationifalinearminimaxestimatorisusedinsteadofaminimaxestimatorisatmost10%andmaybein 102

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AfullviewofXf0,whichisanite(;V)-densesubsetofX0

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Azoom-inviewofthebottomleftcornerofXf0,whichisanite(;V)-densesubsetofX0 Theriskof^x(0),thederived-optimalestimator,asafunctionofx02Xf0

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Theaprioridistribution0,whichisdenedonXf0 ThemaximumriskoftheKalmanFilterinitializedwithzeromeanandcovariance2Iasafunctionof

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106

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108

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TheconcavityofKfollowsfromthefactthatforany0<<1,K(1+(1)2)=inf^x2XK(^x;1+(1)2)inf^x2XK(^x;1)+(1)inf^x2XK(^x;2)=K(1)+(1)K(2);wheretheinequalityaboveisadirectresultof( B ).Eqn.( 9 )impliesthatK(1)K(2)forany01<21.SinceKisdecreasingandconcaveon[0;1],itiscontinuousexceptperhapsatthepoint=1.CertainlyK(^x;1)=K(1)andK(^x;)K()forall01.Thusforany0<1,jK()K(1)jK(^x;)K(^x;1)=(1)sup2R(;^x)r(;^x)(1) 109

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wherethesecondinequalityisadirectconsequenceof( 9 ).ThisimpliesthatK(^x2;1)K(^x2;2)K(^x1;1)K(^x1;2).IteasilyfollowsthatG(^x1)G(^x2).2)By( C ),K(^x1;2)K(^x2;2)0.Thusr(;^x2)+(12)[G(^x2)G(^x1)]r(;^x1):Bypart1)ofthelemma,r(;^x2)r(;^x1).3)By( C ),K(^x2;1)K(^x1;1)0.Thussup2R(;^x1)+1[G(^x2)G(^x1)]sup2R(;^x2):Bypart1)ofthelemma,sup2R(;^x1)sup2R(;^x2). 110

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[1] R.E.Kalman,Anewapproachtolinearlteringandpredictionproblems,ASMETransactions,JournalofBasicEngineering,vol.82,pp.34,Mar.1960. [2] J.Heffes,TheeffectoferronousmodelsontheKalmanlterresponse,IEEETrans.Automat.Contr.,vol.AC-11,pp.541,Apr.1966. [3] T.Nishimura,Ontheaprioriinformationinsequentialestimationproblems,IEEETrans.Automat.Contr.,vol.AC-11,pp.197,Apr.1966. [4] T.Nishimura,ErrorboundsofcontinuousKalmanltersandtheapplicationtoorbitdeterminationproblems,IEEETrans.Automat.Contr.,vol.AC-12,pp.268,Jun.1967. [5] M.Mintz,AnoteonminimaxestimationandKalmanltering,IEEETrans.Automat.Contr.,vol.AC-14,pp.588,Oct.1969. [6] J.M.Morris,TheKalmanlter:Arobustestimatorforsomeclassesoflinearquadraticproblems,IEEETrans.Inform.Theory,vol.IT-22,pp.526,Sep.1976. [7] V.PoorandD.P.Looze,Minimaxstateestimationforlinearstochasticsystemswithnoiseuncertainty,IEEETrans.Automat.Contr.,vol.AC-26,pp.902,Aug.1981. [8] C.J.MartinandM.Mintz,Robustlteringandpredictionforlinearsystemswithuncertaindynamics:Agame-theoreticapproach,IEEETrans.Automat.Contr.,vol.AC-28,pp.888,Sep.1983. [9] S.VerduandH.V.Poor,Onminimaxrobustness:Ageneralapproachandapplications,IEEETrans.Inform.Theory,vol.IT-30,pp.328,Mar.1984. [10] S.VerduandH.V.Poor,Minimaxlinearobserversandregulatorsforstochasticsystemswithuncertainsecond-orderstatistics,IEEETrans.Automat.Contr.,vol.AC-29,pp.499,Jun.1984. [11] J.C.DarraghandD.P.Looze,Noncausalminimaxlinearstateestimationforsystemswithuncertainsecond-orderstatistics,IEEETrans.Automat.Contr.,vol.AC-29,pp.555,Jun.1984. [12] B.I.Anan'ev,Onminimaxstateestimatesformultistagestatisticallyuncertainsystems,ProblemsofControlandInformationTheory,vol.18,pp.27,1989. [13] Y.L.ChenandB.S.Chen,Minimaxrobustdeconvolutionltersunderstochasticparametricandnoiseuncertainties,IEEETrans.SignalProcessing,vol.42,pp.32,Jan.1994. [14] B.I.Anan'ev,Minimaxestimationofstatisticallyuncertainsystemsunderthechoiceofafeedbackparameter,JournalofMathematicalSystems,Estimation,andControl,vol.5,pp.1,1995. 111

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YoavN.LevinbookwasborninTelAviv,Israel,onOctober30,1974.HereceivedtheB.S.degree(magnacomlaude)fromTelAvivUniversity,Israel,in2000andtheM.S.andPh.D.degreesinelectricalandcomputerengineeringfromtheUniversityofFlorida,Gainesville,in2006and2007,respectively.HewaswiththeMotorolaSemiconductor,Herzliya,Israel,andSmartlink,Netanya,Israel,asanelectricalengineer.Hisresearchinterestsincludestatisticaldecisiontheory,stateestimation,signalprocessingforcommunications,andsensornetworks. 114