On the lattice of II⁰₁ classes


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On the lattice of II⁰₁ classes
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Riazati, Farzan
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Copyright 2001


Farzan Riazati


I wish to express my gratitude to my advisor, Professor Douglas Cenzer. I also

wish to thank Andre Nies.


ACKNOWLEDGMENTS ............................. iii

LIST OF TABLES ................................. v

ABSTRACT ... ... .. .. ... ... .. ... . . .. . .. ... vi


1 PRELIMINARIES ........................... 1

1.1 Terminology and Notation ...................... 1
1.2 Existence and Examples ........................ 5
1.3 Basic Properties ......................... 8
1.4 Appearances and Applications .................... 11
1.5 Stone Representation and Computable Boolean Algebras . 15

2 LOCAL PROPERTIES OF H? CLASSES . . . . . .... .. 17

2.1 Post Program and the Lattice Cn . . . . . . .. .. 17
2.2 Principal Ideals of H? Classes . . . . . . . ... .. 17
2.3 The World of Simple IIH Classes . . . . . . ... .. 21
2.3.1 Minimal Extensions of H? Classes . . . . ... ..22
2.3.2 Quasiminimal IIH Classes . . . . . . ... .. 30
2.3.3 Fully Complemented Principal Ideals . . . ... ..33
2.3.4 Where There Are No Complements; Nerode's Theorem 38
2.4 Principal Covering Filters . . . . . . . . ... .. 41
2.5 The Splitting Property . . . . . . . . . ... .. 44

3 GLOBAL PROPERTIES OF II? CLASSES . . . . . .... .. 48

3.1 Definability, and Automorphisms . . . . . . .... .. 48
3.2 Homogeneity and Embeddings . . . . . . . .... .. 51

REFERENCES . . . . . . . . . . . . . . . . ... .. 58

BIOGRAPHICAL SKETCH . . . . . . . . . . . . .... 62


Table page

2.1 Initial principal ideals of ^ ....... .......................18

2.2 Principal filters of * . . . . . . . . . . . . .. .. 22

2.3 Structure of principal ideals of C .. ................... ..32

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


Farzan Riazati
August 2001

Chairman: Professor Douglas Cenzer
Major Department: Mathematics
Effectively closed sets, as modeled by H' classes, have played an important role

in computability theory going back to the Kleene basis theorem [1955]. Many of the

fundamental results about 110 classes and their members were established by Jockusch

and Soare in [1972]. Cenzer et. al [1999] is a short course on IIH classes. The n? classes
occur naturally in the application of computability to many areas of mathematics.

Cenzer and Remmel [1998] is a recent survey with many examples. Minimal and

thin IH? classes were investigated by Cenzer et al. [1993]. This dissertation is a

comparative study of the lattice Cn of H? classes with the lattice of computably
enumerable (c.e.) sets. The work in this thesis concerns the lattice of H? classes
(modulo finite difference), compared and contrasted with the lattice of c.e. sets. The
notion of a minimal extension Q of a class P is defined to mean that there is no
class strictly between P and Q. Previously only trivial examples were known, but

here we give general conditions under which P has a minimal extension. Recently
initial segments of the lattice (that is, subsets of a given set) have been studied. It

was shown, in contrast to the lattice of c.e. sets, that a finite lattice can be realized

which is not a Boolean algebra; in particular, any finite ordinal can be realized. This

thesis announces an improvement of these results by constructing a 11 class P such

that the family of subclasses of P is isomorphic to the smallest infinite ordinal (w).

Also studied are definability of various properties (such as finiteness) and invariance

under automorphism.


1.1 Terminology and Notation
We begin with some basic definitions. Let w = {0, 1,2, ...} denote the set of natural
numbers. For any set E, E<' denotes the set of finite strings (a(0),... ,c(n- 1))
of elements from E. And Ew denotes the set of countably infinite sequences from
E. For a string cr = (u(0), (1),..., u(n- 1)), luall denotes the length n of a. The
empty string has length 0 and is denoted by 0. A string of n many k's is denoted
by kV. For m < i la, a m is the string (a(0),..., a(m 1)). We say cr is an initial
segment of r (written ar -< r) if a = rfm for some m. Given two strings a and r,
the concatenation of a and r, denoted by crUr (or sometimes just ur), is defined
by orT = (a(O),U(1),...,a(m 1),r(O),r(1),...,r(n- 1)), where Ilall = m and
IT|11 = n. We write a-a for a"(a) and aau for (a)-'. For any x E EW and any
finite n, the initial segment x[n of x is (x(0),...,x(n 1)). For a string a E and anyx E E", wewrite a < a-x if u = x [nfor some. For anycr En and any
x E ES, we have aux = (o(O),..., a(n 1), x(O), x(1),...). Given strings ar and r
of length n, we let a r = (ar(O),r(O),..., a(n 1), r(n 1)); if lull = n + 1 and
1 rll = n, then a r = (urn r)-a(n). Given two elements x,y of E-, x y = z
where z(2m) = x(m) and z(2m + 1) = y(m).
We need to code a string a E w pairing function, that maps the pair of natural numbers m, n E w to [(m+n)2+3m+n]
For each a E wk, we let < a >= [< a[(k- 1) >,a(k)], where <>= 0, and < m >= mn
for each mn E w.

A tree T over Ei< is a set of finite strings from E<- which contains the empty
string 0 and which is closed under initial segments. We say that r E T is an immediate
successor of a string a E T if = a'-a for some a E E. Since our alphabets will
always be countable and effective, we may assume that T C w<^. Such a tree is
said to be w-branching since each node has potentially a countably infinite number of
immediate successors. We shall identify T with the set {< a >: oa E T}. Thus we say
that T is computable, computably enumerable, etc. if {< a >: a E T} is computable,
computably enumerable, etc. For a given function g : w said to be g-bounded if for every a E w Thus, for example, if g(cr) = 2 for allao-, then a g-bounded tree is simply a binary tree.
T is said to be finite branching if T is g-bounded for some g that is, if each node of
T has finitely many immediate successors.
Observe that this is equivalent to the existence of a bounding function h such that
a(i) < h(i) for all a E T and all i < Ial|. T is said to be computably bounded (c.b.)
if it is g-bounded for some computable function g. As above, this is equivalent to the
existence of a computable bounding function h such that acr(i) < h(i) for all a E T
and all i < ljaoI. If T is computable, then this is also equivalent to the existence
of a partial computable function f such that, for any a E T, a has at most f(a)
immediate successors in T. A computable tree T is said to be highly computable if it
is also computable bounded. For any tree T, an infinite path through T is a sequence
(x(O), x(l),...) such that x \n E T for all n. We let [T] denote the set of infinite paths
through T.
It is important to note here that we consider a 110 set to signify a subset of w and
in general a 110, E or A set is a subset of w with the appropriate form of definability
in the arithmetical hierarchy [23]. We denote by card(A) the cardinality of the set or
class A. Given two trees S and T contained inw T such that 11r|| < 110a|i < ||T11 + 1}. For two II classes P = [S] and Q = [T], define

the amalgamation of Pand Q, P Q, by P Q = { y :x C PAy Q}. Then it
is clear that P 0 Q =[S T. More generally, define the infinite amalgamation OiSi
to be those strings a such that for each i, (a([i,O]),a([i, 1]),.. .,a([i,j])) E S,, where
j is the maximum such that [i,j] < ]Iall. Then [0iS,] is isomorphic to the direct
product rili[Si].
We also wish to consider the following notion of disjoint union. Given two trees
S and T contained in w I0 classes P = [S] and Q = [T] P E Q = {Ox x P U {l-y : y Q}. It is easy
to see that [S e T] = [S] [T]. Clearly S T is bounded if and only if both S and T
are bounded and similarly for the other notions of boundedness. More generally, the
infinite disjoint union DiQi may be defined to be {(i) y y y Qi} for unbounded
classes Qi. A node a of the tree T C w<` is said to be extendible if there is some
x C [T] such that a -< x. The set of extendible nodes of T is denoted by Ext(T).
Ext(T) may be viewed as the minimal tree S such that [5S] = [T]. A node a E T is
said to be a dead end if a ( Ext(T), that is, if a has no infinite extension in [T].
We are interested in 110 classes in the spaces {0, 1}'(the Cantor space) and wa
(the Baire space). The topology on wu is determined by a basis of intervals 1(a) =
{x C w : cr -< x}. Notice that each interval is also a closed set and is therefore said
to be a clopen set. Moreover the clopen subsets of the Cantor space are just the finite
unions of intervals.
The Cantor-Bendixson derivative D(P) of a compact subset P of {0, 1}" is the
set of nonisolated points of P. Thus a point x E P is not in D(P) if and only if there
is some open set U containing x which contains no other point of P. Equivalently,
x D(P) if and only if there is some closed set U such that U fl P = {x}. Another
useful observation is that, for any compact set P, D(P) is empty if and only if P
is finite. The iterated Cantor-Bendixson derivative D'(P) of a closed set P C X
is defined for all ordinals a by the following transfinite induction. D(P) = P;

D+l'(P) = D(D`(P)) for any a; DA(P)= no<\ Da(P) for any limit ordinal A. The
Cantor-Bendixson rank of a countable closed set P is the least ordinal a such that
D'+'(P) = 0. The (effective) Cantor-Bendixson rank of a point x E X is the least
ordinal a such that, for some H class P, Dc(P) = {x}. For a E {0, 1}<', the interval
I(a) = {x E 2 : or --< x}. We denote by )C(2w) the collection of all clopen subsets
of 2W i.e. the finite unions of intervals, I(rai) U ... U I(on). We also note that for a
closed subset Q of 2w, D(Q) = {x: (VU E KA(2w))(x E U -4 (3y 5 x)(y E U n Q)}.
We refer the reader to Odifreddi [39], or Soare [47] for the basic definitions of
computability theory. In particular let O, be the partial computable functional with
index i and let Oi,s be the computation of 4j for s steps, so that the function @,, is
uniformly primitive computable. We write 0,(o,) -. if (3s)(4e,s(r) -) and ,e(ca) t if
not 0, (a) 4.. The computably enumerable sets are enumerated as We = {n: ,e(n) i.}.
The computable functions of type two, which take both number and function variables
can also be enumerated, as 4). Here we write o(n) to denote the result of computing
e on a number variable n and a function variable x. The result of computing V(n)
for s steps is written O,8(n) and uses only the first s values of x.
Given two sets A and B, we write A write A =T B if both A class of A under =T. We say that A is many-one reducible to B, A is a computable function f such that x E A iff f(x) is in B. We say A is one-one
reducible to B is there is one-one computable function f such that x E A iff f(x) is
in B. We write A =m if A Given a set W C w, define the jump of W, W', to be {e: OW(e)converges} where we
identify W with it's characteristic function. Let 0 be the Turing degree of the empty

1.2 Existence and Examples
The existence of non trivial [ classes is a consequence of the following classic

Theorem 1.2.1 (Konig [30])
An infinite tree in which every node has only finitely many immediate successors

has an infinite branch.
Consider the proof of the above lemma. Let T be such a tree. We define an infinite
branch by induction. We start with uo. Given an with infinitely many extensions
on T, let an,+ be an immediate successor of on with infinitely many extensions on
T. It exists because a, has infinitely many extensions on T, but only finitely many
immediate successors. Thus at least one of them must have infinitely many extensions

on T. An analysis of the proof of the Konig's Lemma provides that :
Corollary 1.2.2 (Kreisel's Lemma)
An infinite computable binary tree has a A infinite branch.
This result has been improved by Shoenfield [44] who proved that there is always
a branch of Turing degree less than 0'. However the next result is much stronger.

Theorem 1.2.3 (Jockusch, Soare [25])
The Turing degrees a such that a' = 0', (low degrees), form a basis for H' classes.

Every H class has an element of low degree.
Recall that [T] denotes the set of infinite paths through a tree T. That is, [T] =

{x E 2w : (Vn)(x[n T)}. For any tree T, the set of extendible nodes of T is defined
by Ext(T) = {a: (3x E [T])(a -< x)}. For a II? class P, which is the set of infinite
paths through some computable tree T, we denote the sub-tree Ext(T) of T by Tp.
The II? class P is said to be decidable if Tp is computable, or equivalently, if P = [T]
for some computable tree T with no dead ends.
The task of showing that a particular class P is H' is often simpler than producing
a computable tree T with P = [T]. It suffices to have an effective procedure which,

given an oracle for a function x P, discovers that x P within finitely many steps,
and runs forever on any oracle x C P.
Definition 1.2.4 An element x of a H' class P is said to be isolated if there is some
a such that P fn I(a) = {x}.
As we shall see, isolated paths are often computationally trivial. In fact, the
computational complexity of the Cantor-Bendixson derivative of a IIH class is an
important feature.
It is often useful to view II classes in 2<" via their complements. There are many
trees representing a particular class P, but there is one tightest representation via
the strings not in P, which brings us to the following definition.
Definition 1.2.5 A subset G of 2<- is called a 2 then r E G. And if both a-O E G and a-1 E G then a G.
Furthermore, G C 2<' is called a 2<' c.e. ideal if G is a c.e. set and is a 2<' ideal.
We have the following correspondence between IIH classes and 2 Lemma 1.2.6 Let P C 2w, Then P Cn - 2<"w \ Tp is a 2<" c.e. ideal.
Suppose P = [T] is a n? class where T is a computable tree. 2 \ Tp is a c.e. set,
since, by Konig's lemma, a E 2w \Tp (3n)(Vr E {0, 1}")aTT T By definition
of Tp, if r E Tp then so are all of its initial segments. Therefore a E 2<' \ Tp implies
that all of a's extensions are too. Finally if a E Tp the some extension of a must be
there too. (Indeed a has extensions of all lengths). In particular, one of a'0 or a-l
must be in Tp by the downward closure property. Therefore 2 Conversely, suppose 2 x E P <= (Vn)(Vs)x[n ^ Tp,. If G is a 2 denote the II? class {x 2w : (Vi)(x[F+j G)}.
II? classes may also be characterized in terms of c.e. ideals and filters in the
countable Boolean algebra C(2-) of clopen subsets of 2".

Let Z(P) = {U E KC(2-) : U n P= 0}, and Up = {u E KC(2w) : P C U}. Observe
that U e I(P) U U UpK, and Z(P) is clearly an ideal, and Up is a filter, for
each P C 2w.
Lemma 1.2.7 The following are equivalent:

i) P is II class.
ii) Z(P) is a c.e. ideal.
iii) Up is a c.e. filter.

i) => ii) Let G = 2w \ Tp, since P is a H? class, G is a c.e. ideal. Let U =
I(Cri)UI(72)U.. .UI(an), then U E Z(P) 4= (Vi < n)o E G. ii) = iii) This follows
from U E Z(P) -= U E p. iii) => i) Forany a, wehave a E G = I(acr) E Z(P),
so G is a c.e. ideal, which makes P a H1 by the previous lemma.
There are IIH? classes with Tp of every c.e. degree. Given any c.e. set, we let
0'1 in the sub-tree of the extendible nodes, by branching off the limit path 0' when
and only when n E. Let E be any c.e. set, define a 11? class by setting Tp
to contain 0' for every n, and for every m satisfy; n Em ==> 0"1m E Tp.
Thus [Tp] = {0fO} U {0'1' : n E}. This will be an important example for later
developments, when the above c.e. set E is a maximal set.
The fundamental example of H? classes is the separating class of two separable
c.e. sets. The sets separating two disjoint c.e. sets A and B form a [I? class:

C E SA,B 4 (Vx)[(x E A -4 x E C)&(x E B -+ x C)].

Explicitly, a computable tree whose branches are exactly the members of SA,B is the
following: x E TA,B 4==* x is a sequence correct up to Iixll where 'x is
correct up to stage s' means that for every i < ]xa|, if i E As then x(i) = 1, and if

i E B, then x(i) = 0. In other words, we seal off a branch of TA,B as soon as we
discover it is incorrect. Note that TA,B has an infinite branch if and only if A and B
are disjoint. Moreover an infinite branch of TA,B is the characteristic function of a

set separating A and B.
Simpson poset is used to give a universal example of H' classes.
Definition 1.2.8 Simpson Poset of degrees

Simpson order << is defined in [45] as ; b << a
every infinite tree T C 2
particular, 0 << a means that every nonempty II class of sets has an element of
degree < a. There is a universal 11 class U C 2w defined as;

U = {x E 2w: (Ve)[x(e) (e)]}.

This class U is a class of sets which separate {e: 0,(e) = 0} and {e: 0,(e) = 1}.

Obviously U J 0 has no computable member and is quite big. It also has the property
that the degree of elements of U coincide with the degree of complete extensions of

Peano arithmetic. It has also been shown that, for any degree a, 0 << a if and only
if there exists f U of degree < a. Thus the degrees of elements of U are exactly
the degrees >> 0, which means that each element of U can compute an element of

any nonempty H' class, hence the universality of U.

1.3 Basic Properties
When a II? class has no computable element, the elements are generally of lower
computational complexity. Jockusch and Soare [25] shown that a IIn class without
computable members has cardinality 210. Since every isolated infinite branch of a
computable tree is computable, if there are no infinite computable branches every
branch splits, and the number of infinite branches is 21. Jockusch and Soare [25]
also proved that A IIH class without computable members is meager. It follows that
singleton II? classes are topologic statements about their unique members.

Lemma 1.3.1 For any x E 2-, the following are equivalent:

i) x is computable.

ii) {x} is a [II class.
iii) The Cantor-Bendixson rank of x is zero.

Basis Theorems are the computability theorist's Choice principles. These the-
orems are often informally stated as: "Every simply definable class has a simply
definable member" Kleene observed that, given the set of extendible nodes Ext(T)

as an oracle, one can compute an infinite path through T (if one exists) by letting;

x(n) be the least k such that (x(O),..., x(n 1)) E Ext(T). For an arbitrary com-
putable tree, Ext(T) is E. The type of "simply definable element" depends on the

complexity of the "simply definable class".

Theorem 1.3.2 (Kleene, Kreisel-Shoenfield, Cenzer-Remmel)
For each H? class P C W,

i) P has a member computable from some E' set;

ii) If P is bounded, then P has a member computable from 0";

iii) If P is computably bounded, then P has a member computable from 0';
iv) If P is decidable, then P has a computable member;

v) If P is highly polynomial-time decidable, then P has a p-time member.

Forcing with IH classes, is another witness to the richness of the structure of
subclasses of a H class under inclusion. This suggests similar questions to Post's
program for the c.e. sets. In case of computably bounded HI classes, more can be
said about the "simply defined element":
Theorem 1.3.3 (Jockusch and Soare[24])

Each nonempty c.b. 11H class P contains:

i) an element of low degree.

ii) an element of c.e. degree.
iii) two elements whose degrees have infimum zero.

iv) an element of hyperimmune-free degree.

Part (i) above which is known as the, Low Basis Theorem, is proved (like parts
(iii) and (iv)) by forcing with nonempty H' subclasses of P. This is also interesting

for methodological reasons. It is the first time the "information content" of a ll

class P, is used to achieve results on its "computational content". (Once again more

information about the II class P, would provide finer results).
Theorem 1.3.4 Kleene Basis Theorem

Let P be a countable 11H class of functions. Then

i) P has a hyperarithmetic member.
ii) If P is bounded, then P has a member computable from 0'.

iii) If P is computably bounded, then P has a computable member.

The following result may not look like a Basis Theorem, but it shows that nonempty
computably bounded 11 classes are really "computably bounded". This result is used

in second order arithmetic.
For any sequence Co, C1i,... of noncomputable sets and any nonempty c.b. 11
class P, has a member x such that no C; is Turing reducible to x.
The last and the most exciting Basis theorem is from Cenzer, Downey, Jockusch
and Shore[7] which relates the computational complexity of an element x of a II? class
P, to its topological standpoint in P.

Definition 1.3.5 A Hl? class P is thin, if every subclass Q C P is relatively clopen
in P, that is there exists a clopen set U such that Q = P n U.
Definition 1.3.6 A 117 class M, is called a minimal H' class if for each H' subclass
P C M, either M \ P is finite or P is finite.
Lemma 1.3.7 (Cenzer et al. [2])
For any thin H' class P and any element x of P, x is isolated in P if and only if
x is computable.
Lemma 1.3.8 (Cenzer et al. [7])
Let P be a H1 class. If P is thin, and D(P) is a singleton, then P is minimal. If
P is minimal and infinite, then D(P) is a singleton.
Theorem 1.3.9 (Cenzer et al. [2]) Existence of Thin Classes.
For every recursive ordinal a, there is a thin Hl' class Pa with Cantor-Bendixson
rank a, moreover P, may be taken to be the set of paths through a computable tree
with no dead ends.
Variuos construction can be carried out under general conditions specified in the
next result.
Theorem 1.3.10 (Cenzer, Downey, Jockusch and Soare[7])
Let P be a H' class of sets and suppose x E P. If x has rank 1 in P, then x If P is decidable, then x decidable, then x'
1.4 Appearances and Applications
Logical theories and their extensions are the first applications. Since the class of
sets separating two disjoint c.e. sets is a nonempty H1 class, so are:

i) the set of consistent extensions of a given consistent theory.
ii) the set of complete extensions of a given consistent theory.

There are converses known to these results:
Theorem 1.4.1 (Hanf [22])
The class of degrees of members of a given II? class coincides with the class of
degrees of complete extensions of some finitely axiomatizable first-order theory.
Theorem 1.4.2 (Scott [43)
If T is a consistent extension of PA, the sets computable in F form a basis for
IHl classes.
Theorem 1.4.3 Solovay [49]
The following are equivalent:

i) a is the degree of a consistent extension of PA
ii) a is the degree of a complete extension of PA
iii) D(5 a) is a basis for II classes.

The set P(T) of computable extensions of a given axiomatizable theory F in first-
order logic is always a IIH class. For the theory F = PA (Peano Arithmetic), this
provides the historically first example of a nonempty IIH class with no computable
member. It can be shown that PA does not have maximal consistent extensions of
low degree, c.e. degree, hyperimmune-free degree, etc.
Theorem 1.4.4 (Ehrenfeucht [19])
Each II? class P C 2' can be represented in the form P(F) for some axiomatizable
propositional theory F.
For a decidable theory F, P(r) is a decidable IIH class and every decidable II?
class of sets can be represented by the set of complete extensions of a decidable
propositional theory. It follows that every decidable consistent theory has a decidable
complete extension.

A minimal decidable H class P, corresponds to a decidable theory T with exactly
one undecidable complete extension and such that any axiomatizable extension of T

is a principal extension of theory T.
The complete extensions of an axiomatizable theory can be viewed as the maximal
ideals of the c.e. Lindenbaum Boolean Algebra. Thus every decidablee) H1' class of
sets may be represented as the family of maximal ideals of a c.e. computablee) Boolean

The next set of examples including challenging open questions are in Commutative
rings. The collection of all prime ideals of a c.e. commutative ring with unity composes
a c.b. H class. The example of Boolean rings shows that any c.b. H class of sets may

be represented as the set of prime ideals of some computable ring.
Theorem 1.4.5 (Friedman, Simpson and Smith [20])
Each II' class P of separating sets may be represented as the set of prime ideals
of a computable commutative ring with unity.
Problem 1.4.6 Does every c.b. H1 class represent the set of prime ideals of some

computable commutative ring with identity?
Graph theory, is another major application of 11 classes. The best source on
problems in computable graph theory, is the Handbook of Recursive Mathematics
[12]. Some properties of thin and minimal NO classes, have interesting representations

in terms of graph coloring.
If a thin HO class P is the set of k-colorings of some computably enumerable
graph G, then for any computable coloring f of a computable subgraph H, the set
of extensions of f to G is a H1' subclass of P and thus there is a finite subgraph G1
and a coloring g, of Gi such that the extensions of f are exactly the extensions of

gi. For a minimal class P and a computable coloring f of a computable subgraph H,
we see that either there are only finitely many k-colorings which extend f or all but
finitely many k-colorings of G extend f.

Partially ordered sets of finite width and their decomopositions, is another solution
set representation problem.
Theorem 1.4.7 (Dilworth [9])
Every finite poset of width k can be decomposed as the union of k chains.
Where the width of a finite poset is the cardinality of the largest anti-chain.
There is a natural dual to this theorem which says that every poset of height k can
be covered by k antichains. The family of all decompositions of a given computable
poset as the union of k chains (or anti chains) (k fixed) can be represented as a c.b.

II' class. For the special case when P is the class of separating sets of a disjoint pair
of c.e. sets, this has been answered by Cenzer and Remmel in [12], and independently

by Hirst.
The solution sets to many problems on the computable continuous functions on
polish spaces, are c.b. II classes. The connection here is the following observation:
The graph of a computably continuous function on a Polish space is always a 'II
class. And for some Polish spaces, computably continuous functions have decidable

II1 graphs, and each function with a II? graph is computably continuous. For each
computably continuous function f, the following sets are II classes.

i) The set of zeros of f.
ii) The set of points where f attains a max or min.
iii) The set of fixed points of f.
iv) the complement of the basin of attraction of a computable periodic point of
f, i.e. the Julia set of a computably continuous function f.

The first two problems can represent any II? class in the given space. The fixed
point problem can represent any II? class except for the space [0, 1] where the class

must have a computable member. And the last problem can represent any class which
is bounded and has a computable max and min element.
Ramsey theory, index sets, are among the other applications. The recent paper

of Cenzer and Jockusch [9], has a detailed analysis of the current state of research on
fl? classes, including Ramsey theory.

1.5 Stone Representation and Computable Boolean Algebras

Theorem 1.5.1 Every ideal on a Boolean algebra can be extended to a prime ideal.

The Prime Ideal Theorem, is a weaker version of the axiom of choice, which is

often used in many proofs in algebra and topology in place of full AC (compacti-

fication theorems, Hahn-Banach theorem,...). It also gives the following important

representation theorem for Boolean algebras.

Theorem 1.5.2 Every Boolean algebra is isomorphic to a field of sets.
Let B be a Boolean algebra. We let S = {p : p is an ultrafliter on B}. For every

u E B, let Xu be the set of all p E S such that u E p. Let T = {Xu : u E B}
Let us consider the mapping 7r that 7r(u) = Xu. Clearly 7r(1) = S and 7r(0) = 0.
It follows from the definition of ultrafilter that 7r(u.v) = 7r(u) n 7r(v), 7r(u + v) =

7r(u) U 7r(v), 7r(-u) = S \ 7r(u). thus 7r is a homomorphism of B onto .T(and T is a
field of sets). It remains to show that 7r is one to one. If u v then using the prime

ideal theorem, one can find an ultrafilter p on B containing one of these two elements

but not the other. Thus 7r is an isomorphism.

Definition 1.5.3 The Stone space of a Boolean algebra B is the above space S with
the topology given by the base F in the above theorem.
With the above terminology, the Stone space is a compact Hausdorff space with a
base of clopen sets. The Boolean algebra B is isomorphic to the algebra of all clopen
sets of its Stone space. If the Boolean algebra B is countable then B is isomorphic
to the Boolean algebra RC(P) of relatively clopen sets of a closed class P C 2-.


Downey et al. [7] have proved an effective versions of these results and used it to

transfer results on H[ classes to results on Boolean algebras which can be obtained as

the quotient of a computable Boolean algebra by a c.e. equivalence relation. Among

others they have discovered the interpretations of thinness and Cantor-Bendixson

derivative in computable Boolean algebras.


2.1 Post Program and the Lattice n
It was first observed in Myhill [34] that the collection of all c.e. sets { We}e16 forms
a lattice under inclusion, =< {We}e'; C>. The collection of all H classes of 2W
forms a lattice under inclusion C relation and is denoted by n =< {Pe} .E ; C>.
For a recent work on the Medvedev reductions see Cenzer and Hinman [8]. In this
chapter we focus on the lattice Cn.
Post [38] initiated studying the relationship between the Turing degree of a c.e.
set, and its structure as a set with respect to set inclusion. For example Post defined
a c.e. set A to be simple if its complement is infinite, but contains no infinite c.e.

set. Post's Program was to find connections between the set theoretic structure C of
a c.e. set and its Turing degree. We consider analogous questions in the lattice Cn,
that resemble the Post's Program. We will compare and contrast with the analogous

notions for the c.e. sets.

2.2 Principal Ideals of H Classes
The IIH classes form a distributive lattice with smallest and greatest element, such
that the clopen sets are the only complemented elements. As in the case of , these
properties are far from characterizing the structures n and Cf. We will study the
structure of the initial segments, and also we look at what structures may be realized
as substructures of the lattice of H classes.
Definition 2.2.1 n(P) is the principal ideal of Cn generated by the H' class P, i.e.

Cn(P) = {R E nC : R C P}. We also denote by C(P) the lattice of II subclasses of

P modulo finite differences. In fact C(P) is the quotient lattice of n(P)/1, where
Z is the ideal of finite H' classes.
Definition 2.2.2 [P, Q] is the interval of n between the H' classes P and Q, i.e.
[P,Q] = {R En: P C R C Q}. Similarly, we use [P,Q]*, for the quotient [P,Q]/I,
for the interval of P and Q in Ln.
An ideal or filter itself forms a lattice under the induced operations. The structure
of principal ideals C (P) can be highly varied. However the following important cases
are already known.
For a minimal H? class M, n(M) is isomorphic to the collection of finite, cofinite
subsets of w, and C*(M) 0 {,1}. On the other hand n(P) for a thin 11' class P
is lattice isomorphic to a subalgebra of the Boolean algebra of clopen subsets of 2',
consisting of those clopen sets that have nonempty intersection with the closed set
P, it follows that C(P) is also a Boolean algebra.
The situation is summarized in the following table:

Table 2.1: Initial principal ideals of *
H? Class P The Principal Ideal Generated by P
P minimal C(P) 0 {0,1}
P thin n(P) Boolean Algebra

The following idea is central in characterization of finite lattices that can be re-
alized as initial segments in Cn. In fact the next two theorems classify some of the
principal ideals of the lattice n.
Definition 2.2.3 The lattice (, <) satisfies the dual reduction property if for any
a,b E 4, there exists a, > a and b, > b such that a, V b, = 1 and a, A b, = a A b.
Reduction property has roots in descriptive set theory of pointclasses in Polish
spaces. Many well known effectivizations of the classic pointclasses enjoy the reduc-
tion property. Some interesting collections of sets in computability theory satisfy
either the reduction property or its dual form, the separation property.

Proposition 2.2.4 (Herrmann[unpublished])
For each 117 class P, the lattice n(P) satisfies the dual reduction property.
Let P1 and P2 be (nonempty) 131 subclasses of P and, for i = 1,2, let T, be a
computable tree such that P, = [Ti] is the set of infinite paths through Ti. We define
computable trees Si such that T Cg S. with Si l s2 = T fn T2 and S1 U S2 = {0,1}<`
and with Q, = [Ti]. It will follow that Qi n Q2 = PI n P2 and that Q1 U Q2 = {0,1}.
For the first condition suppose that x Qi f Q2, then x[n E i fn S2 for each n, so
that x[n E T1 n T2 for each n, and therefore x PI n P2. For each x, we have that
for each n, either x[n E St or xrn E S'2. Thus without loss of generality x[n E S1
for infinitely many n. Since S' is a tree, x[n E S'1 -+ x[m E S'1 for m < n, so that
x[fn E S1 for all n and therefore x E Qi. The definition of the trees Si is by recursion
on the length of a E {0, 1}<'. First put the empty string in both S1 and S2 since it
is in T1 n T2. Now assume by induction that for strings a of length < n, the following
hold; aE 1 U S'2 and a E 5si n s2 == a Ti n T2. Now for r = aoO or a-l, there
are four cases:

i) If r E T1 n T2, then we put r Si nl S2.
ii) If r T1 \ T2, then we put r E S1 \ S2.
iii) If r E T2 \ T1, then we put T E S2 \ S1.
iv) If r T, U T2, then we consider whether a E S1 or S2.

If a E S2 \ S1, then we put r S2 \ S and otherwise, we put r E S \ S2. It follows
that in each case, if r E Si, then a E Si, so that each 5, is a tree. The conditions
(i) and (ii) follow from the construction by induction on the length of a. The proof
ends with the observation that the pair Q1, Q2 dually reduce the pair of P1, P2.


The following result, is helpful in determining the isomorphism type of Ln(P),
for certain 11' classes.
Theorem 2.2.5 (Cenzer and Nies [10])
For any finite distributive lattice L with the dual reduction property, there exists
a II class P such that I2 (P) is isomorphic to L. Furthermore, the theory of IC*(P)

is decidable.
Let P be as in the above theorem for some finite lattice L. The decidability of
the theory of L*(P), is a major point of difference between the lattice of c.e. sets,
and the lattice of 1l classes.
Nies [37] proved that, in the lattice , the theory of each interval which is not
a Boolean algebra interprets true arithmetic and is therefore undecidable. However,
according to the above theorem, there are initial segments in 4n, of the form [0, P] =
Ln(P), which are not Boolean algebras, but with a decidable theory. The decidability
of 4n(P) is proved using a result of Lachlan[31], which shows the theory of Ln(P),
is many-one reducible to the theory of the finite lattice L*(P).
On the other hand, the decidability of P, would imply the undecidability of the
theory of LrIn(P). Cenzer and Nies [10] have recently shown that for a decidable II
class P, if 4n(P) is not a Boolean algebra then the theory of 4n(P) interprets true
arithmetic and is therefore undecidable. Thus we have the following.
Theorem 2.2.6 Let P be a II? class such that for some finite lattice L with the dual
reduction property, such that C*(P) L, then the tree Tp is not computable, i.e. P
is not a decidable IIH class.
The observation made in the above two theorems makes the construction of a ll
class realizing a finite lattice a challenging task.
Corollary 2.2.7 For each finite ordinal n < w there exists a H? class P, such that
L(P) has order type n.


Let L = {OL = a, < a2 < ... < an = 1L} be the lattice of order type n. To

apply the above Cenzer-Nies theorem, we observe that L satisfies the dual reduction

property. For aj, a, E L with ai < aj we have ai A an = ai = ai A aj and a V an =

an = IL.

Thus there are initial segments of order type n, for any given finite ordinal. The

following theorem shows there are initial segments in L* of infinite order type. Build-

ing on the proof given in Cenzer and Nies [10] we have shown in Cenzer and Riazati

[14] that:

Theorem 2.2.8 The lattice C* has principal ideals *(P) of order type w + 1.

2.3 The World of Simple 11 Classes

Myhill [34] also asked whether there is a maximal c.e. set in the inclusion ordering

modulo finite sets. A co-infinite c.e. set A is maximal if there is no c.e. set W such

that W n A and W n A are both infinite. Equivalently, A is maximal iff A* is a

coatom(maximal element) in *. Soare [48] mentions the importance of maximal

c.e. sets for the following reasons: they are coatoms of * and hence the building

blocks for more complicated lattices of supersets; they were the ultimate realization

of the Post's search for sets with thin complements; their degrees are exactly the high

degrees. Maximal c.e. sets are the simplest of the simple sets.

Atoms of the lattice *, and classes with at least near-atomic structure in the

lattice, turn out to be analogues of the simple-like sets in the lattice of c.e. sets .

The fact that dual notions to simple c.e. sets are of low 2n-rank, and rather ad hoc

distinctions between different notions of simple c.e. sets, would lead to modifications

of these dual notions in the lattice fn of IIH? classes. There are characterizations of

simple c.e. sets in the lattice *:

Table 2.2: Principal filters of S*
c.e. Class A the principal filter generated by A
A maximal *(A) {0,1}
A quasimaximal C*(A) finite
A hyperhypersimple *(A) Boolean Algebra
A r-maximal *(A) with no complements
A without maximal supersets $*(A) dense

We study IIH classes analogous to quasimaximal and r-maximal c.e. sets. Princi-
pal ideals generated by these classes are not in general finite or non-complemented
respectively. We will also consider H classes with finite or non-complemented CL(P).

2.3.1 Minimal Extensions of 11 Classes
Recall that an infinite II class P is minimal if for each II[ class R E Lr(P),
either R is finite or P n R is finite.
We have observed that, if P is a minimal 110 class then C*(P) {0, 1}.
We focus on possible formations of II? classes as finite principal ideals in LC.
Minimal classes are the atoms of L and one such possible formation. The definition
of minimal IIH? classes has non-trivial extensions to principal filters of Cn:
Definition 2.3.1 A IIH? class Q is a minimal extension of a II? class P C Q if for
every n? class R E [P, Q], either R n P is finite or Q n R is finite.
Remark. If M is a minimal class with M n P infinite, then P U M is a minimal
extension of P. On the other hand, adding a copy of any minimal class M to any
interval not intersecting P, yields a trivial minimal extension of P which has a new
limit path. Thus we make the following.
Definition 2.3.2 A H? class P is said to admit Q as a proper minimal extension
(P Cmin Q) if Q is a minimal extension of P, and D(P) = D(Q).
We now show that certain II? classes have proper minimal extensions. We need
to use the limit lemma, that any function f that is computable in 0' has a uniformly
computable approximation {f8}s such that lim,+oofs(x) = f(x).

Theorem 2.3.3 Each II? class P with a single limit point A with A < 0', admits a
proper minimal extension.

Let P and A be as described. Let As be the uniformly computable approximation
given by the Limit Lemma (see Soare p. 57). Since A P = [S], we may assume
that A'[s E S for all s. If it is not, simply find the longest initial segment a of As [s
which is in replace As [s with any extension r of a which is in S and has length s.
For any fixed n, there exists an m such that A [n -< A3 [s for all s > m and it follows

that the modified version of As also extends A [n for all s > m, so that we still get A
as the limit of the sequence A3.

The minimal extension Q of the class P is obtained by adding to P an infinite
sequence B, of new isolated paths such that A [n -< Bn for each n. This immediately
ensures that Q can have no new limit paths. (If some B E Q is a new limit path, then

we must have B(n) 5 A(m) for some least m. Then the interval I(B[m + 1) cannot

contain Bn for any n > m and can only contain finitely many of the isolated paths
from P, since B was not a limit path of P. But this contradicts the fact that every
neighborhood of a limit path must contain infinitely many elements of the class.)
This means that Q will be a proper minimal extension of P if we can show that it is
a minimal extension.
To ensure that Q is in fact a minimal extension, we need to show that for any II
class Pe, if P C Pe C Q, then either P, P is finite or Q Pe is finite. To accomplish

this, we require that for each e, there exists some k(e) such that if k(e) < i < j and
Bi Q, then Bj E Q. Thus if P, P is infinite, then it must contain some Bi with
i > k(e) and hence must contain all Bj with j > i.
The construction of the tree T is in stages s, using a priority argument. We will
define a computable sequence of threshold numbers n(s) and at each stage s, the
following are defined.

The threshold number n(s) and the tree Ts = T f {0, 1}(S).
For i < s, the s-approximation /3O of Bi such that /3' S.
T will be a computable tree since for a C {0, 1}', we have a T ==* a E T'.
The new isolated paths Bi will be defined by B, = lims-,.oo /.
There are two types of requirements for the construction.
To ensure that As -< Bi, we have: Rj: A8[i -< Of,:
The Requirements: To ensure that for every II class Pe with P C Pe C Q, either
Pe \ P is finite or Q \ P, is finite, we have, for each j> i > e, the requirement:
R,,i,j: if/37' E T, then /3j E Te.
Note that for each e, this requirement only needs to be satisfied for sufficiently
large i, that is, for i > k(e), where k(e) is a function computable in 0', but not
necessarily computable, which will be shown to exist later. Priority is assigned to the
requirements as follows.
Rj has priority over Rk if j < k.
Rk has priority over Re,i,j if k < i.
Re,i,j has priority over Re',i',j' if either
(1) 1<' or (2) i = i' andj Requirement Ri requires attention:
at stage s + 1 when /O3 does not extend AS+'1 [i.
Before describing the action to be taken for this requirement we note that since
P = [S] has only one limit path, every node a E S has an extension which is not in
S. The action for this and the other requirements are all going to require defining
nodes oa, and 7, as follows. Let a, be the shortest and then lexicographically least
extension of As+l [n(s)-(1 As+1(n(s))) which is not in S and let y, be the shortest
and then lexicographically least extension of As+l fn(s) + 1 which is not in S. We are
always going to define n(s + 1) to be the maximum of {f I|I, |IIII|}.

The action to be taken when Ri requires attention is the following. We need to
redefine /3 and also define /3 for the first time. Define f+l to have length n(s + 1)
and extend a, by a string of '0's and similarly define 13+ to have length n(s + 1)
and extend -y, by a string of '0's. For each j < s different from i, let/3J+1 have length
n(s + 1) and extend /3 by a string of '0's. The tree TS+l contains all nodes from S
of length n(s + 1) as well as the nodes /3+' for all k < s.
Requirement Re,i,j requires attention at stage s + 1 when

(i) As+'ri -< Bj3O,
(ii) 1js0 ( Te and 03'O E Te, and
(iii) for all d < e, if /3'-0 Td, then /3;O0 Td.

The action to be taken when Re,i,j requires attention is the following. We let /3s+
be the sequence of length n(s + 1) which extends /3j&-0 by a string of '0's, we let
O'+1 be the sequence of length n(s + 1) which extends a, by a string of '0's and we
let /3+1 be the sequence of length n(s + 1) which extends 7, by a string of '0's. For
each k < s different from i and j, let 3k+1 have length n(s + 1) and extend O by a
string of '0's. The tree TS+l is defined as above to contain all nodes from S of length
n(s + 1) as well as the nodes /O+1 for all k < s.
Initially, we let n(0) = 0 and let To = {0}. At stage 1, we have TV = {0, (0), (1)}
and we have 0 = (1 A'(0)).
At stage s + 1 > 1, we take action on requirement of highest priority which needs
attention. If no requirement needs attention, we just let n(s+1) = I||a,|, let /3s+1 = a,
and for each j < s, let /3j+' have length n(s + 1) and extend /J by a string of '0's.
The tree TS+' is defined as above.
It is clear from the construction that for each s, /O,/,... ,/3 and As[n(s) are all
distinct nodes of length n(s) in TP.

We have to show that each requirement is eventually satisfied and that for each
s, lims)co3 = B exists and belongs to Q = [T].

A key concept in showing this convergence is the notion of the e-state of a finite

or infinite path. An infinite path B has e-state (co, c1,...) where Ce = 1 if B E Pe
and c, = 0 otherwise. For the finite path /3, we define the e-state to be (Co, ci,..., c,)

where ce = 1 if and only if 3 E Te. In either definition, the e-states are ordered

lexicographically, so that (co,..., c,) is lower than (cr,..., c) if ce
least such that they are different.

An important observation is that whenever we take action on any requirement
Re,i,j at stage s + 1 > i, we lower the e-state of 3, that is, the e-state of f0+1 is lower

than the e-state of /3f. The action taken change Ce itself from 1 to 0 and, for any

d < e, Cd can only decrease because of the final clause in the definition of requiring
attention. On the other hand, if we take some other action at stage s + 1, then the

e-state is either the same or lower, since 03+1 will then be an extension of f, so that

/3+1 Te implies O?, E Te for any e.
This demonstrates the following claim.

CLAIM 1: For each i, the e-state of /3i converges.
The next step in showing that the construction converges is the following.

CLAIM 2: Each requirement only requires attention at a finite number of stages.
Proof of Claim 2: This Claim is proved by induction on the priority. Suppose
that all higher priority requirements have been satisfied, so that our requirement has

highest priority from some stage s on. There are two cases.
For the requirement Ri, we may assume that s > i and that s is large enough
so that At[i = A[i for all t > s. If Ri ever requires attention at some stage t > s,
then we take action and get A[i -< /0'. The only other action which could affect this
requirement is action on some Re,i,j for some j > i. But this action will make Oi'3+

an extension of Ar+1 [n(s) and hence still an extension of A[i.

For the requirement Re,i,j, we simply take a stage s large enough such that the
e-state of 3i has converged and all lower priority requirements have ceased requiring
attention. If Re,i,j ever required attention after stage s, we would take action and
thus lower the e-state of /03, a contradiction.
Once the requirement Ri and the e-state of fh converge, then Bi can converge.
CLAIM 3: For each i, the sequence /3f converges to an infinite path B, E Q.
Proof of Claim 3: By the previous claims, we may take s large enough so that the
e-stage of /3 has converged and so that Ri no longer requires attention. Then after
stage s, any action taken only extends /3f by a string of '0's. Thus Bi = 3'-0w.
Now we can determine the structure of our Hl class Q = [T].
CLAIM 4: Q=PU{Bn : n < w}.
Proof of Claim 4: To see that each Bn is in Q, take s such that B. = /,'0,w for
all t > s. Then for any t > s, Bn [n(t) = /3 Tt, so that B, [T] = Q.
Now consider an arbitrary B Q. For each s, B[n(s) E Ts, and is thus either in S
or equal to some #3. If the former happens infinitely often, then B E P, thus we may
assume without loss of generality that Brn(s) = '3s,) for some sequence i(s). Now if
there is a fixed I such that i(s) = i for infinitely many s, then B = Bi, as desired.
Otherwise, there must be infinitely many stages s+1 such that i = i(s+l1) 5 j = i(s).
But this means that /3? --< /3+1, and this can only happen when we act on some
requirement Re,i,j with e < i < j. But this makes i(s + 1) < i(s), which can only
happen finitely often.
Next we check that the B, will approach A in the limit.
CLAIM 5: For each i, A[i -< Bi.
Proof of Claim 5: By the previous claim, Bi = ,/jO, where s is large enough so
that R, never requires attention after stage s and by the argument in Claim 2, large
enough so that A"[i = A i. It follows that Af[i = As [I -< Bi.

We now consider a stronger notion of convergence of the e-states, necessary to
obtain the minimality condition.
CLAIM 6: The e-states of B, converge to a limit.
Proof of Claim 6: This means that for each e, there exists some k(e) such that
for anyj>i> e, Bi Pe P, : Bj EP.
Definition of k(0):
Case I: For all k, Bk i Po. Then let k(0) = 0.
Case II: There exists k such that Bk E P0. Then let k(0) be the least such k. Now
suppose that j > k and consider a stage s large enough so that the 0-states of both
Bj and Bk have converged and such that Bj = j0syw and Bk = /,30w and suppose
by way of contradiction that Bj Pe. Then A[j -< /3yjO and we have (3'-0 E To
and f#j0 To, so that requirement Ro,k,j would need attention and we would act to
put f0+' To, contradicting our assumption of convergence.
Definition of k(e + 1):
Case I: For all k > k(e), Bk i Pe+i. Then let k(e + 1) = k(e).
Case II: For some k >_ k(e), Bk E Pe+i. Then let k(e + 1) be the least such k.
Now suppose that j > k and consider a stage s large enough so that the e + 1-states
of both Bj and Bk have converged and such that Bj = 3J0-w and Bk = 0I0
and suppose by way of contradiction that Bj ( Pe+i. First observe that we have
/3#'0 E To and /30 (O To. Next let d < e and suppose that Pj'O i Td. Then
Bj Pd and j > I > k(e + 1) _> k(e), so that Bk Pd either, which means that

Ok'0 Te. Thus requirement Re+,k,j would need attention and we would act to put
k+1 T,+1, contradicting our assumption of convergence.
Finally, we can demonstrate that Q is a minimal extension of P.
CLAIM 7: For any e, if P C Pe C Q, then either Pe P is finite or Q Pe is

Proof of Claim 7: Suppose that Pe P is infinite and let k(e) be given by Claim
6 so that for m > n >_ k(e), Bm P, if and only if B,, E Pe. Then there is some
n > k(e) such that Bn E Pe and therefore Bm E P, for all m > n. Thus Q P, is
infinite as desired.
CLAIM 8: D(P)= D(Q).
Proof of Claim 8: Let A be a limit point of Q which is not a limit point of P. First
suppose that A ^ P. Then (since P is closed) there is a clopen set U with A E U
such that P n U = 0. Then Q fl U is a H1 subclass of Q which is disjoint from P and
contains A. Since A is a limit point of Q, Q n U must be infinite. Now consider the
IIH class R = P U (Q n U). Since R \ P is infinite, it follows from the definition of
minimal extension that Q \ R = (Q \ U) \ P is finite. We can obtain a [ class M
disjoint from P with Q = P U M, as follows, let (Q \ U) \ P = {B, ..Bt}, for each i,
choose a clopen set Vi with Bi Vi and Vi n P = 0 and let M = (Vi U... U Vk U U) n Q
. If M is not minimal, then clearly P U M is not a minimal extension of P. If M is
minimal, then P U M is not a proper minimal extension. Next suppose that A E P
but is isolated. Then there is a clopen set V with A E V such that Pf V = {A}. But
Q n V is infinite, since A is a limit point of Q. It follows as above that (Q \ V) \ P is
finite, and once again we can define a 11' class M, disjoint from P with Q = P U M
This completes the proof of Theorem.
Remark. If P Cmin Q then [P, Q]* {0,1}, the trivial Boolean algebra.
Remark. Let Q be a minimal extension of P, such that Q 5 M U P for any
minimal II class M. Then Q is a proper minimal extension.
Theorem 2.3.4 Let P be an infinite II class. Then P does not admit a decidable
proper minimal extension.
This follows from our splitting theorem 2.5.3 below. Given P C Q with Q a
decidable 11 class, we consider the following two cases:

Case I: If P is complemented, then Q \ P is a 11 class and Q = P U (Q \ P) thus
Q \ P must be minimal and Q is not a proper minimal extension of P.
Case II: If P is not complemented, then by splitting theorem there exists a 11'
class P1 with P C PI C Q with P \ P and Q \ P1 both infinite, so that Q is not a
minimal extension of P.
Remark. This implies that structure of decidable classes is much simpler.

2.3.2 Quasiminimal H11 Classes
Definition 2.3.5 Let M =< Mo, M1, ..., M-i > be a finite sequence of H1 classes.
If Mo is a minimal class, and Mi Cmin, M+l, Then M is called a minimal chain, of
length n.
Definition 2.3.6 A 117 class P is called a pure extension of a class M, if for some
minimal chain < Mi : i < n >, M = Mo and, P = Mn. We will refer to terms of
arbitrary minimal chains as pure Ho classes.
Lemma 2.3.7 For each R11 pure class Q, the lattice C(P) is finite.
Since Q is pure, then Q is the union of a minimal chain M1,..., Mn = Q. We
show by induction that LC(Q) is finite. For n = 0, M0 is a minimal class, and thus
C(Mo) has two elements. Now suppose that IL(Mk) is finite with < 2k+l elements
and Mk+j is a minimal extension of Mk. Let P be a 117 subclass of Mk+m, and let
P1 = P n Mk C Mk. There are at most 2k+l choices for P1, modulo finite difference.
Now we claim that for each P1 C Mk there are at most 2 subclasses P of Mk+j
with P nl Mk = P1, so that card(C(Mk+i) < 2k+. Given P and P1 = P fl Mk, let
P+ = P U Mk, then Mk C P+ C Mk+i, so there are two cases. First, P+ \ Mk could
be finite, in which case P C* Mk and P = P1. Second, Mk+m \ P+ could be finite, in
which case P =* P1 U (Mk+i \ Mk). In general Mk+j \ Mk may not be a 117 class, so
we can only say that card(I (Q)) < 2k.

Definition 2.3.8 A 11 class Q is said to be a linear extension of a 11' class P, if
the interval [P, Q] is linearly ordered.
Observe that Q is a linear extension of P if and only if there do not exist distinct
II? classes P1, P2, P3 in [P, Q]* with P1 =* P2 U P3, and P2 incomparable with P3
modulo finite differences.
Remark. Not all minimal extensions are linear. Recall the 5-element lattice.

{0, Po, Q1, Q2, P1} where Qi and Q2 are complements in [Po, Pf1], but < 0, Po, Qi, P1 >
is still a minimal chain. This example shows that finiteness of L(P) does not imply
that P = U{M1, M2, ..., Mn}, for finitely many minimal II classes M1, ..., Mn. While
in the lattice of c.e. sets modulo finite differences, S*, finiteness of *(A), a principal
filter of $*, would impose an anti-chain structure on c.e.-supersets of A, moreover A
would have to be the intersection of finitely many maximal c.e. sets. This example
shows that we need the following definition.
Definition 2.3.9 A 11 class P is quasiminimal if P is the union of finitely many
pure H' classes.
Remark. A decidable quasiminimal H' class is a union of disjoint minimal classes.
This follows from theorem 2.3.7
Theorem 2.3.10 A H' class P is a finite union of minimal classes if and only if
12(P) is a finite Boolean algebra.
Suppose first that P = Pi U P2 U ... U Pn is a finite union of minimal II? classes.
We may suppose without loss of generality that P, : Pj for i j, since if Pi n P1
is not finite then Pi =* P, n P, =* Pj and Pi U P, would also be minimal. Now if
Q is a 1 subclass of P, let F = {i : P, n Q is infinite }. Since each P, is minimal,
Pi n Q =* P, for i F and of course Pj n Q =* 0 fori F. Thus Q =* UiEF Pi and

has complement Q =* UjFPj in L (P). On the other hand, suppose that L(P)
is a finite Boolean algebra and let P1, P2, ..., P, be the atoms. It is immediate that

P =* PI U... U Pn. Now Pi U... U Pn = Po C P and we may obtain P = Po as follows.

Let P \ Po = {AI, ..., Ak}, for each i, take an interval Ui with Ai E Ui and Ui n Po = 0
and let U = U1 U ... U Uk. Then P n U = {AI, ..., Ak}. Now simply adjoin this l'
class P n U to P1.
Theorem 2.3.11 For each H1 class P,
P is a quasiminimal HI class if and only if I(P) is finite.
First suppose P is a quasiminimal, that is P = Q1 U ... U Qn, with each Qi a pure
IH' class. By the lemma, for each i, L*(Qi) is finite, say ki many elements. Then
any subclass Q of P can be written as Q = (Q n Qi) U ... U (Q n Q,n). Since there
are < ki many choices for each i this shows that card( (Q)) <_ k, x k2 x ... x k,
and is therefore finite. Now suppose that C*(P) is finite, we prove by induction that
P is quasiminimal. If Card(IC(P)) = 2, then P is minimal. Next suppose that
Card(IC(P)) = n + 1, and let R1, R2,..., Rk be coatoms in C*(P). Then for each i,
P is a minimal extension of R, and, by induction (Since Card(IC(Ri)) < n), each
Ri is the union of some minimal chain, Mij, Mi,2, ..., Mi,k(i). Then Mi,, ..., Mi,k(i), P
is again a minimal chain and P is the union of those minimal chains.
Corollary 2.3.12 For any H' class P, if C*(P) is finite, then L(P) is a Boolean
Definition 2.3.13 For each quasiminimal IH? class P, the quasirank of P is defined
to be the cardinality of the lattice C(P).
Table 2.3: Structure of principal ideals of Cn
II Class P The Principal Ideal Generated by P
P minimal C(P) 0 {0,1}
P is quasiminimal C(P) finite
P is finite union of minimals *(P) finite Boolean algebra
P thin Cn(P) Boolean Algebra

Non pure quasiminimal 11 classes are interesting for their splitting properties.

We will elaborate on this and other splitting properties in what follows.

2.3.3 Fully Complemented Principal Ideals

In E, the notion of hyperhypersimple c.e. sets can be singled out via different

approaches. The Owings Splitting Theorem implies that the principal filter generated

by a hyperhypersimple c.e. set is fully complemented (a Boolean algebra) and vice

versa. Clearly, a coinfinite c.e. set is hyperhypersimple if and only if for every c.e. set

B, if A C B then A U B3 is c.e..

Definition 2.3.14 Splitting property for c.e. sets. A c.e. set S has the splitting

property if every c.e. nonrecursive set A can be split into two nonrecursive c.e. sets

B and C such that B C S.

Remark. Hyperhypersimple sets have the splitting property.

Remark. The c.e. sets with the splitting property form a filter in C.

Remark. The possible isomorphism types of C*(A) for A hyperhypersimple are

exactly the E' Boolean algebras.

Lemma 2.3.15 The theory of hyperhypersimple sets is decidable.


Since the lattice of all hyperhypersimple sets modulo finite sets is relatively com-

plemented, by the same proof which shows that C*(A) is a Boolean algebra for A

hyperhypersimple, the result then follows from Ershov [18].

Definition 2.3.16 A Ho class P is thin if for each Hl class Q E n(P), there is a

clopen set U C 2W such that Q = U n P.

The principal ideal generated by a thin H' class P is a Boolean algebra. The

first example of a thin II class is due implicitly to D. Martin and M. Pour-el in [33].

They constructed an axiomatizable, essentially undecidable theory T such that every

axiomatizable extension of T is finitely axiomatizable over T. It is easy to see that

the class of complete extensions of such a theory T is a thin II? class, and it is perfect
because it contains no computable element.
The first explicit example of a thin IIH? class is due to S. Simpson(unpublished).
Countable thin HI? classes of arbitrary rank, including minimal classes, were con-
structed in Cenzer et. al. [7].
Remark. If P is a thin 1? class if and only h(P) is a Boolean algebra.
Lemma 2.3.17 (Cenzer et al. [7])
For any thin II? class P and any element x of P,
x is isolated in P if and only if x is computable.
Lemma 2.3.18 (Cenzer et al. [7]) Let P be a IIH? class.

i) If P is thin, and D(P) is a singleton, then P is minimal.
ii) If P is minimal and infinite, then D(P) is a singleton.

Theorem 2.3.19 (Cenzer et al. []) Existence of Thin Classes.
For every recursive ordinal a, there is a thin II? class P, with Cantor-Bendixson
rank a, moreover P, may be taken to be the set of paths through a computable tree
with no dead ends.
Perfect thin IIH? classes were constructed by Simpson and are related to supermin-
imal profinite groups by the work of R. Smith [46]. If P is a perfect thin IIH' class
with 0 = Q C P then Q is also perfect thin. Each such class can be split into two
perfect thin classes by a clopen set. Let P be a minimal and thin II? class. Then
D(P) = {A}, and A is non computable. If B E D(P), and P a thin I? class, then B
has to be non computable. Therefore the notion of Cantor-Bendixson rank happens
to capture the computational complexity of members of a thin IIH? class. On the other
hand minimal classes (that are forced to have unique limit path) with computable

limit path are not thin, and thus generate a rich structure as their principal ideal, as
we shall see below.
Indexing H1 classes, corresponds to a fixed enumeration of the c.e. sets.
Definition 2.3.20 A Hl class P is said to be the eth HO class, i.e. P = PF, if and
only if, x E Pe = (Vn)(xan r We).
Index sets for IIH classes were developed in Cenzer-Remmel [12]. It is important
that we can express Pe uniformly as the class of infinite paths through a computable
tree Te.
Lemma 2.3.21 There is a computable relation R C w x 2< such that, for each e,
the set T, = {: R(e,)} is a tree and Pe = [Te].
We have x E Pe == (Vn)(x[rn we). Let R(e,a) <= (Vr -< o)(r ( We).
First we check that Te as defined above will be a tree. Suppose a E Te and al -< a.
Then for any r -< oa, we also have r -< a so r We. Thus ai E Te. Next we check
that x E PFe == (Vn)xrn E Te. Suppose x E Pe and let a = xrn,r -< ao. Then
r = xrm for some m < n, so that T we. Thus xrn E Te. Suppose next that
(Vn)x[n E Te, then certainly for each n, xrn V We, so that x E Pe.
The complexity of local complementation in n, C*, is used to find the isomor-
phism type of these structures.
Arslanov [1] proved there is no coinfinite c.e. set A such that the filter C(A) is
effectively complemented. Similar results holds in Cn(P).
Theorem 2.3.22 There is no infinite I1 class P such that Cn(P) is effectively com-
plemented, i.e. such that, for some computable function f,

P. C P => P () U P, = P A Pf() n PF = 0

Let Ph(.) = P, f P. Then fh is a computable function, and hence it has a fixed

point z for which P h(z) = P,, so that P, is a complement of P n P, a contradiction.

Arslanov et. al. [2] showed that there is no coinfinite c.e. set A, such that, for
some function f : that the corresponding result holds for C(P).
Theorem 2.3.23 There is no infinite H1 class P s.t. for some function f :T )C, the
principal ideal LC*(P) is f-complemented, i.e.

P C P = Pf(,) U P =* P A Pf(,)nP =* 0.

For each set X, cx lemma, if and only if cx = limsoog(x, s) for some computable function g. This can
be applied to show that iff
such that Wf(.) =* W,. The rest is similar to the case of Cn(P) .
Thus for each H? class Q E n(P) the complement of Q in CL(P) is not even
computable in the complete c.e. set C.
Possible isomorphism types of LCn(P) for P thin, can be contrasted with the prin-
cipal filter generated by a hyperhypersimple c.e. set. Lachlan [31] has proved that

the possible isomorphism types of C*(A) the principal filter generated by a hyperhy-
persimple c.e. set, are exactly the E0 Boolean algebras. The c.e. sets are closed under
union and intersection uniformly effectively, namely there are computable functions
f and g such that Wf(,,y) = Wx U Wy, and Wg( ,y) = W, n Wy. Also one might
observe that Wx C* Wy is E-complete in x and y, (Wy is cofinite -= W C* Wy)

also W_ C Wy is II-complete, making it necessary for the Boolean algebra to be at
least E0. The lattice operations n and U are also computable in r. In general the


relation P, C Py is also 11' complete and P, C* Py is also E complete. However for
a thin 11 class, n(P) has a simpler representation.
Theorem 2.3.24 For each [? class P,
If P is thin then the lattice Cn(P) is a c.e. Boolean algebra.
The Boolean algebra L(P) may be realized as a quotient of the Boolean algebra
of clopen subsets of 2' intersected with the H class P. In other words, for Q, R
as above, there are clopen sets U, V such that Q= P N U, R = P n V. For each
pair of clopen sets U, V, let U =-p V <=' U fn P = V n P. First observe that
for U = I(oai) U ... U I(ak), U n P = 0 4== a, Ext(P) A ... A ak i Ext(P)
which is a E condition on U. Then u n P C V n P == (U\V) n P =0, and
(UnP=VnP) #= 7UnPCVnPandVnPCU nP. Thus thepartialorder
and the equivalence relation are both E.
Remark. The lattice C(P) is certainly a E' Boolean algebra, since "U n Pe is
finite "is E and U n P C* V n P 4= (U \ V) n P is finite. Since the relation
"U N P, is finite is XO-complete, it seems that C(P) should not in general be a So
Boolean algebra.
However, we can still show that for a thin class P, n(P) has a complement
function computable in 0".
Theorem 2.3.25 For each thin H class P, there is a function f for all x,
Pr C P = Pf(x) U Px = PandPf(x) n P = 0.

We observe that the condition P, n Py = 0 is 1H and the condition P, U Py = P
is II0, so that both can be verified using a 0"-oracle. Given that P is thin, r(P)
is complemented, so there exists for every x, a P-complement Py for P, such that

(P, nPy)U P = P and Pyn(P, nP) = 0. Define f(x) to be the least y which satisfies
the two conditions.
Definition 2.3.26 A II? class Q is a thin extension of P if for each R E [P,Q],
there is a clopen set U such that R = PU (Q n U) = Q n (Pu U).
Remark. Our example of a non pure quasiminimal IIH class would show that,
thinness is not necessarily equivalent to, [P, Q] being complemented, that is, for any
R E [P, Q], there exists some S E [P, Q] such that R n S = P and R U S = Q.
Clearly if M is a thin class with M \ P infinite, then P U M is a thin extension of
P. Thus we say that Q is a proper thin extension of P if in addition there is no thin
class R such that Q = R U P.
Remark. The existence of proper thin extensions follows from that of minimal

2.3.4 Where There Are No Complements; Nerode's Theorem
The opposite notion to that of thin IIH classes, is a II class P where no non trivial
element of C(P) is complemented. The above example of a non pure quasiminimal
class as the join of two proper minimal extensions, is one such II class, for which
C*(P) has 5 elements, 0, a minimal class M, two of its minimal extensions, Qi and

Q2 and Q1 u Q2.
Definition 2.3.27 An infinite IIH? class P is 7r-minimal if CI(P) has no non trivial
complemented elements.
Our terminology is similar to that of r-maximal c.e. sets. An r-maximal c.e.
set A has an r-cohesive complement, i.e. A is infinite and can not be split into two
infinite parts by a recursive set. Let us recall equivalent descriptions of r-maximal
c.e. sets;
Lemma 2.3.28 For each c.e. set A, the following are equivalent;

i) The filter Ce*(A) has no nontrivial complemented elements.

ii) A is r-cohesive.
iii) A is infinite, and there is no pair of c.e. sets whose union is w and whose
intersection is A.

Lemma 2.3.29 (Robinson[42])
There exists an r-maximal set which is not maximal.
The following theorem of Nerode characterizes the nonprincipal ultrafilters of
the Boolean algebra of computable sets, REC*, as the intersections of REC* with
the principal filters of P(w) (modulo the finite sets) generated by the recursively
indecomposable sets:
Theorem 2.3.30 (Nerode, Shore [36])
The non principal ultrafilters of REC* are exactly those of the form UEC = {X* :
A C* X E REC}, where A is an r-cohesive set.
Or equivalently A is an r-cohesive subset of w -= {X* : A C* X E REC} is
an ultrafilter of REC*. Nerode's theorem suggests similar characterizations for the
notion of 7r-cohesiveness in the lattice C*.
Definition 2.3.31 Let .F C Ln, for each fl' class P, the collection V) = {Q* : P C*
Q E F} is called the .F-covering filter of P.
Definition 2.3.32 An infinite Ho class P, is said to be K-cohesive if for every clopen
set U, either U fn P is finite or U nf P is finite.
Lemma 2.3.33 For each flo class P. If P is 7r-minimal, then P is K-cohesive.
Suppose P is not K-cohesive. Choose a clopen set U such that both U n P and
U n P are infinite. But U n P and U n P are both fl' classes and provide non trivial
complemented elements of *(P).

In other words, for 7r-minimality, if we let FT be taken to be the clopen subsets

of 2', unfortunately such a natural analogy fails to capture all 7r-minimal classes. In

fact in C:
Theorem 2.3.34 For every IIH class P the following are equivalent:

i) P is K-cohesive.
ii) {K: P C* K E /C(2w)} is an ultrafilter.
iii) P is a 11' class with a unique limit point.

i) --+ ii) Assume that P is K-cohesive. the collection V1p is clearly a filter for any
P. Let the clopen set U be given. If U n P is finite then P C* U so U E V^p. If U l P
is finite, then P C* U so U Vcp .
ii) -+ iii) Suppose by way of contradiction that P has 2 limit points A, B. choose
a clopen set U so A E U and B U. Then U l P is infinite, since it has a limit point
A, and likewise U n P is infinite.
iii) -+ i) Assume that P has a unique limit point A and let U be a clopen set.
Without loss of generality suppose A E U. Then U nl P has no limit point and is

therefore finite.
The result of Nerode, suggests a different way to characterize nonprincipal ultra-

filters of C, or at least a new type of ultrafilter of L. This follows from switching
from clopen indecomposability to decidable indecomposability. Recall that, a 11 class
P is said to be decidable, if P = [T], for some computable tree T without dead-ends.
The collection of all decidable IIH classes is denoted by DEC. The lattice of LDEC
modulo the ideal of finite H' classes will also be denoted by CDEC*.

2.4 Principal Covering Filters
The notion of an ultrafilter is suitable for the Boolean algebra of REC*, but needs
to be weakened for the lattices Cn, ECE,* "CDEC*, which are not complemented.
However the notion of a filter and its closure properties still makes sense. But we will
replace the notion of ultrafilter with the notion of prime filter.
Definition 2.4.1 For each FT C TP(2), and each l1 class P, a filter of the form
VPF = {Q* : P C* Q E F} is called Prime if for each pair of fH classes Q, R E FT,
Q U R Vp implies that Q E Vr or R E V-.
Remark. If Vp is prime, then P is 7r-minimal, since a pair of complements Q,
R in L*(P) would violate the conclusion of the primality. However, the II class
with the 5-element lattice, discussed above is ir-minimal, but Vp is not prime, since

Qi U Q2 E Vp, but neither Qi nor Q2 is in the filter VpF.
Definition 2.4.2 A IIH class P is called J-Cohesive if the filter VDEC = {Q* : P C*
Q E DEC} is a prime filter of DEC*.
Lemma 2.4.3 For each H? class P, VDEC C V'. If P is 8-cohesive then P is K-
Theorem 2.4.4 For each H1 class P:

i) If VDEC is prime then Vp> is prime.

ii) If V~p is prime then for some A E P, D(P) = {A}.
iii) If VJDEC is prime then CEC*(P) has no nontrivial complemented elements.

All clopen sets are decidable 11' classes, this proves i). To see ii) we observe
that for the Boolean algebra KA(2w), prime implies ultrafilter, so the previous theorem
applies. For the last part, If Vp is prime then P can not be split by elements of Jr,
that is we can not have fl' classes P and Q in F such that P f Q and P n R are

non trivial complemented elements in C*(P). (Since we would have P C* Q U R but
neither P C* Q nor P C* R.)
Corollary 2.4.5 For each H' class P; If Card(D(P)) > 2 then VDEC is not prime.
Theorem 2.4.6 For each fl class P, with D(P) = {A}. If the filter Vp is not
prime then A is computable, and the filter VDEC is not prime.
1 00
Since Vp0 not prime, we can assume that P admits a splitting into infinite IIr
classes Q1,Q2 with Q1, Q2 VP'. Thus P C* Qi U Q2 and Qi n Q2 =* 0. We can
modify Q1, Q2 to get exactly P = Qi U Q2 and Q, n Q2 = {A}. For the first part we
get Q1 U Q2 C P, by replacing Qi with P n Qt. To get P C Qi U Q2, consider the finite
set F = {A1, A2,..., Ak} = P \ (QI U Q2). These are all isolated in P, and therefore
computable. So F is a IIH class. Now replace Qi with QI U F. Each infinite 11H class
P fl Qi, i E {1, 2} has to contain some limit point, and that has to be the unique limit
point of their superclass P. Thus A E Qi n Q2. But the Hl class Q fn Q2 is finite
and every element of a finite I1 class is computable. We obtain Qi n q2 = {A} as
follows. Let D = Qi n Q2 \ {A} = {Bf1, B2,..., Bm}. Each B, is isolated in P, so we
can get a clopen set U such that U n P = D. Now replace Q2 with Q2 fl U.
Now suppose there are computable trees S1i, S2, such that Qi = [5'i],Q2 = [S2],
and P = Qi U Q2 and Qi n Q2 = {A}. We use a technique from the E1 separation
theorem, to construct R1, R2 E CDEC* such that P = R1 U R2, with R1 n R2 = {A},
and moreover, Qi c Ri, Q2 C R2.
We will define decidable trees TI1, T2, such that R, = [T1] and R2 = [T2]. For
a -< A, we put 0a E T1 U T2. For every node oa A, we must decide if a E Ext(Ri) =
T1, or a E Ext(R2) = T2 or none. Let p cr with ||p|l = n such that p -< A,
and p'ao(n) : A(n), (where ar branches off the limit path). Now by the assumption
Qi n Q2 n I(p-a(n)) = 0, so by Konig's lemma there exists I such that p-oa(n) has
no extension of length I in si 5'2. Compute the least such I and decide a as follows.


First suppose 1|o,| < I. If a has an extension of length 1 in Si we put ao in T1. If
c has an extension of length I in S2 we put a in T2. Next suppose ||al7 > 1. Then

for each i, we put a E Ti, only if aol E Si. Clearly T1 and T2 are decidable trees,
and R1 n R2 = {A}. Let us check that Qi C Ri. Suppose x :7 A and x E Qi. Then
by assumption x Qi-i. Let p be the largest common initial segment of x and A
and let I be the least such that p-x(n) has no extension of length I in S1 n S2. Then

x[l E Si, hence x[l E T, and therefore x E Ri.
Remark. If P is decidable with P = [T], then we can modify the definition of
T, and T2 to make P = R1 U R2 as follows. For the case when Io|| > I just put
or E Ti <-=' arI E Si and 0r E T.
This remark essentially proves that:

Theorem 2.4.7 For each decidable II? class P, with D(P) = {A}, if P is 7r-minimal
then Vpo is prime.

Remark. If P is a minimal and thin II? class, then C*(P) is just the two element
Boolean algebra so Vp1 is prime, and D(P) = {A}.

When the limit point is not computable, non thin minimal classes, and non min-
imal thin classes, have non trivial complemented principal ideals. The following
theorem of Lachlan [31] relates three notions of r-maximal c.e. sets of integers, the
hyperhypersimple c.e. sets, and maximal c.e. sets.

Theorem 2.4.8 (Lachlan [31])
A c.e. set is maximal if and only if it is both hyperhypersimple and r-maximal.

This suggests that the corresponding notion to r-maximality, namely the notion
of 7r-minimality, should be intimately related to the computability of the limit paths.
Or at least in the case of minimal II classes. This is the case at least for decidable
II classes, as we shall see in the following theorem.
The following theorem is a lattice theoretic characterization for non computability
of the limit path of a decidable II? class.

Theorem 2.4.9 Let P be a ?l class with D(P) = {A}. Then

i) The filter Vpo is prime -== A is not computable.
ii) For P decidable, and prime Vp1, if L(P) is non trivial, then C*(P) is infinite.

For the first part, if the filter Vp1 is not prime, then A is computable by the

above theorem. On the other hand, suppose that A is computable. Then {n :

A[nU(1 A(n)) V Ext(T)} is a computably enumerable set and must be infinite

since A E D(P). Enumerate E as ol, 72,..., and let P1 = P \ Un-aI(o2n+1). Then

P C P1 U P2, but P \ P1 and P \ P2 are both infinite.
For ii) suppose that the filter Vp1 is prime, and that C*(P) is not trivial, and let

P0 be a proper II subclass of Po. Thus of course Po has no complement in CI(P),

so by the Splitting Theorem L*(P) is infinite.

The above theorem suggests the importance of the following II classes.

Definition 2.4.10 A H class P is said to be CB-computable if the set D(P) is


Hence, finite and non thin minimal ll' classes are all CB-computable. And in the

light of the above theorem, some ir-minimal [1' classes are not CB-computable 1H'


2.5 The Splitting Property

The fact that each infinite computable set of integers splits into two infinite com-

putable sets is rather trivial, but useful in characterizing the Boolean algebra of

computable sets up to isomorphism.

Theorem 2.5.1 (Freidberg-Muchnik Splitting Theorem)

Every noncomplemented A E splits into two disjoint noncomplemented c.e. sets

B, C e such that A= B U C.

This theorem has extensions to nontrivial principal filters (A) of :
Theorem 2.5.2 (Owings Splitting Theorem)
For any c.e. set D, every noncomplemented element of E(D) can be split into two
noncomplemented elements. That is given a c.e. set A such that A U D is not c.e.,
there are disjoint c.e. sets B and C such that ; A = B U C, and B U D and C U D

are not c.e.
Owings splitting theorem may be compared to the following result.
Theorem 2.5.3 Splitting Theorem for Decidable IIH classes
For each non-complemented Po E n(P), where P CEC, there exists Pi E
[P0, P] such that P \ PI and P1 \ Po are both infinite, and furthermore Pi is non-

Let P = [T], Po = [To] for some computable tree T with no dead ends, such that
Po has no complements in Cn(P) (or in C*(P)), and so P \ Po is infinite. Let the
infinite sequence {o, rl, T2, ...} enumerate the computably enumerable set T\Ext(To).
Define by recursion the new sequence {an : n G w} such that :

0`O := TO;
an+, := Tj where j = pim.[(m > i) A (on = ri) A (rj {lao, a U,...,0n})].
(That is, an+, is incompatible with oi for each i < n).

CLAIM: Card({am :m E w})= No.
In other words the sequence never becomes stagnant, i.e. there is always new
proper choices for Tj in the above construction.
If {am : m E w} = {ao, ..., On}, then every element of T \ Ext(To) is compatible
with one of {0, oo, ..., on}. For each k < n, there are just finitely many predecessors
to each oan. Let A C {o, ..., n, -...} be the finite set consisting of all such predecessors,
namely A = {rj : (30 E {ao, ..., an})(rj -< o)}. Let J = A U {aO, ..., -rJ, For each

x E P \ Po there exists a Tj such that rj -< x and Tj is compatible with one of the
{ao,i, ..., 0n}. Then there are two cases:

i) rj -< Ok for some k, 0 < k < n, which implies (rj A) A (x E I(Tj)).
ii) Ok Tj for some k, 0 < k < n, which implies that x E I(Uk).

Therefore P \ Po = P n K, where K = UJ .7I(p) is a clopen set in C(2w). But
this would imply that P n K is a complement of Po in Cn(P), which ends the proof
of the claim.
Now consider the infinite computable set of incompatible elements:

S= {o, ..., On, ...} which is a subcollection of T \ Ext(To). Let us let Pi =
P \ UJnE I(U2n). Then P1 is a 11' class. We observe that for each n, I(orn) contains
an element of P \ Po. In fact, since T is decidable and the sequence {1, : n G W} is
computable there exists a uniformly computable sequence {Xn : n E w} of elements
of P \ Po, with X, C I(aE). Moreover, it follows from the incompatibility of aUm
and o,, that rn n =- Xm : Xn. Thus PI \ Po contains infinitely many elements
X1,X3, XS,.... On the other hand P \ P1 is infinite since it contains Xo, X2, X4, ....
Let P2 = P \ UI(a2n+i), and observe that P = P1 U P2 and P1 n P2 = Po. Now if P1
had complement Q, so that P1 U Q, = P P1 n Q, = 0, and P2 had complement Q2,
then Po = P1 n P2 would have complement Q1 U Q2, thus at least one of P1 P2 has
no complement.
Corollary 2.5.4 If P E 'EC, and P is not thin, then L(P) is infinite.
Let P be a decidable class with a proper subclass Po which is not complemented.
By the splitting theorem, there is a subclass P1 with Po C* P1 C* P with PI also
not complemented. Applying the theorem again, we get P2 between P1 and P and
by repeating the process, we get Po C P1 C P2 C ... C P, with each inclusion proper.


Proof: (alternatively)
We can embed a copy of * in [P0, P]*, by taking Qe = P \ UnfEWI(o,,). This map
will be discussed in the next chapter.
Restating this corollary, we can say that for any decidable IH class P, if C(P)
is finite, then LC(P) is a Boolean algebra. This was shown by Cenzer and Nies by a
different method.


3.1 Definability, and Automorphisms

Definition 3.1.1 A property 0 of II classes is said to be invariant in n if and
only if it is invariant under all automorphisms of Cn.
Definition 3.1.2 A property ?p is called definable if there is a formula of one free
variable (with no parameters) in the language of lattice theory which defines the class
those classes in Ln having the property 4, with the usual interpretations for relation
Since every automorphism must preserve C, if a property is definable then it

is invariant. A cardinality argument seems to suggest there are invariant but non
definable properties. But first let us see what properties are definable.
Lemma 3.1.3 The property of being clopen is definable in n.
The clopen sets are the complemented objects. Then P is clopen if and only if

(3Q)(PvQ=1 & PAQ=O).

Lemma 3.1.4 The property of being thin is definable in n.
A 11H class P is thin if and only if n(P) is complemented, that is

(VQ)[Q < P -* (3R)(R < P & Q v R = P & Q V R = O)].

Lemma 3.1.5 the property of being a singleton is definable in n.

Singletons are the atoms in n, so P is a singleton if and only if


Corollary 3.1.6 For each n w, the property of having cardinality exactly n, is
definable in n.
D. Cenzer and A. Nies recently announced the following definability result.
Theorem 3.1.7 The property of being finite is definable in Cn.
Conjecture 3.1.8 For each computable ordinal p, the property of being of Cantor-
Bendixson rank p is invariant.
Theorem 3.1.9 The property of being of Cantor-Bendixson rank < 1 is definable in

Using the Cenzer and Nies theorem, A flI class P has the above property if and
only if [(VQ)(VR)[(Q V R= 1 & Q A R = 0) (FIN(Q A P) V FIN(R A P))].
Theorem 3.1.10 The property of being minimal is definable in both Cn, C, using
the definability of finiteness.
A II? class P is minimal if and only if [P >* 0 & -(0 <* Q A Q <* P)].
Theorem 3.1.11 The property of being a minimal class with a computable limit point
is definable in n.
The formula defining such classes, is P is minimal & V)p is prime, which is

(VQ)(VR)[P <* Q U R -+ (P <* Q V P <* R)].

Theorem 3.1.12 (Cholak et. al. [15])
The property of being perfect thin is invariant.

Definition 3.1.13 The Downey degree of a L' class P = [T] is defined to be the
Lemma 3.1.14 The Downey degree of decidable II classes is 0 and vice versa.
If for a HI class P with P = [T], dTuring(Ext(T)) = 0 then P can be repre-
sented as the collection of infinite paths through a computable subtree of T, namely
Ext(T) with no dead ends, and thus P is decidable. Now if P = [T] for some com-
putable tree T that has no dead ends, then Ext(T) = T is computable, and thus
dTuring(Ext(T)) = 0.
Conjecture 3.1.15 The Downey degree is not invariant.
Definition 3.1.16 A minimal H? class P with D(P) = {A} is said to be Standard,
if for each n, there is at most one path x E P with x[n = A[n but x A.
Lemma 3.1.17 For each standard IIH class P, with limit point A, we have A and the family of isolated points of P is uniformly computable from 0'.
Let P = [T]. We can compute A as well as the sequence {Bn}n"e if isolated paths
using Ext(T). First observe that every node a such that a'0 and a-l are both in
Ext(T), lies on A. Thus A is in fact the union of these nodes.
Let a0 < a1 < ... enumerate the set of nodes a such that both Ext(T). Then {a"n}ne is computable from Ext(T), and for any n, A(n) = a,+i(n).
Now Bn is just the unique extension of a different from A, that is, for any m > Ila,|I,
the sequence Bn [m can be computed from Ext(T) as the unique extension r E Ext(T)
of a-(1 A(n)).
The following result was announced by D. Cenzer and J. Remmel:
Theorem 3.1.18 Cenzer [9] The property of being standard is invariant.

3.2 Homogeneity and Embeddings

The general question of homogeneity for a lattice L is to find sublattices that are

automorphic images of the lattice L.

Definition 3.2.1 A lattice C is said to be upward(downward) homogeneous at point
AeC =

C C(A), where C(A) is the principal filter(ideal) generated by A.
Definition 3.2.2 The problem of homogeneity for a lattice C is the complete deter-

mination of the members A of the lattice, at which the lattice is upward(downward)


Definition 3.2.3 A lattice L is said to be computably upward(downward)
homogeneous at point A E L = C C- C(A), where C(A) is the principal fil-

ter(ideal) generated by A, via a computable isomorphism as 1.
Definition 3.2.4 The problem of computable homogeneity for a lattice C is the com-

plete determination of the members A of the lattice, at which the lattice is computably

upward(downward) homogeneous.

There are interesting results on the external homogeneity of the lattice . By

considering the relativized lattice E'.
For the lattices of fl? classes, that we will see downward homogeneity fails. But

there are still downward embeddings that are not isomorphisms.

Lemma 3.2.5 There are homeomorphic l classes that are not automorphic.

Let P, Q be H? classes with D(P) = {A}, D(Q) = {B}, where A is computable
but B is not computable. Then {A} E Cn(P) would have to be mapped to a subclass
of Q under each automorphism, but it is both an atom and meets every infinite
subclass of P, whereas there is no such class in Cn(Q).
With the terminology in the section on splitting property, we state some properties
of the filter of superclasses for trivial cases.

Lemma 3.2.6 For each clopen HI' class P 5 2w, Vp' n.
In fact, we show that Up = {Q : P < Q} Cn and the mapping carries over
to show that Vp' C. The isomorphism will follow from the obvious fact that
2w \ P 2w. That is, once we have a homeomorphism h : 2- -4 2` \ P, we simply
map Q in L to P U h[Q].
Lemma 3.2.7 For each pair of non-clopen 110 classes P, Q, H V1.
We show that Up = {R: P C R} is isomorphic to UQ = {R: Q C R} as follows.
First use the technique of the splitting theorem to effectively enumerate incompatible
nodes {pn}nEw not in Tp and {n}nE, w not in TQ such that 2" \ P = UnlEI(pf,) and
2W \ Q = UnI(on). Now given R D P with R = [S], define the tree T by letting
r E T == (Vn)(Va)[o since for r E TQ, we never have a-oa = r. For each n, F(R) contains an'x -=, R
contains r7,x.
Corollary 3.2.8 For each nonempty H' class P there are at most two isomorphism
types for Vp.
Theorem 3.2.9 (Cenzer and Nies [9])
n n 1
There is no isomorphism between Vg' and VIOL which is computable in 0".
Definition 3.2.10 For each H' class P, let fp be the function fp : 2 such that f(a) is defined to be the number of extensions of a in P.
For example if P is standard, then fp[2 Definition 3.2.11 We define 1-degree, of a 1l1 class P, to be the Turing degree of
the counting function fp of P, i.e. 1-degree(P) = dTuring(fp).
Theorem 3.2.12 There are H1 classes of the same Downey degree but different 1-

Let E be a c.e. noncomputable set, and P = {0"'} U {0Onl : n E L} U {olm''0 :
n E Em+i \ E}. Then fp(0Ol) = 2 if and only ifn E E. Thus 1-degree(P) = deg(E).
But it is clear that P is decidable, that is given any node ao, a E Tp if either

i) (Vn < II|la|)(a(n) = 0, or
ii) (3m < IIo|)(Vn < I|a|o)(o7(n) = 0 <==> n < mn) or
iii) 3m,n < IIall)(Vk < IHIll)(o,(k) = 1 4== n < k < m).

We now show that the lattice of 11' classes is algebraically at least as complicated
as the lattice of c.e. sets under inclusion. This is done by embedding in n(P).
Theorem 3.2.13 There exists a n? class P, with D(P) = {10}, such that (, D) is
isomorphic to the principal ideal riCn(P) generated by P.
Let P be the HIo class {0'"} U {0' 1 : n E w}. For each e E w define a i-subclass
of P by letting Qe = {0"} U {Onlw0 : n We} C P. The required embedding would
map We -+ Qe. We observe that Qa C Qb == Wb C Wa :
If Qa C Qb, for each n E We the infinite branch 0l10w E Qa, thus 010f E Qb, or
equivalently n E Wb. The converse is similar. We also observe that {Qe : e E w} is
exactly the collection of all infinite HI' subclasses of P. To see that these are the only
infinite subclasses of P, for each infinite Q E n(P), let W = {n : Ollow V Q}. But
with this definition W E , therefore for some e C w, W = We and this implies that
Definition 3.2.14 For future references, we call the II? class P in the above theorem
the 0w-class. We also call the corresponding embedding as lo.
Corollary 3.2.15 The above embedding induces an embedding from E* onto n(P).

Theorem 3.2.16 The lattice (E, D) may be embedded in Cn(P) if: P is a non thin
decidable [1I class, or P is an infinite 1-decidable H' class.

In each case, it is possible to construct a sequence on of incompatible elements
of 2<-, with (Vn)P n I(oTn) : 0. Now let Qe = P \ UnEw. I(a,). This will be an
embedding as in the above argument. In the case of 1-decidable 11a class P1, there
are infinitely many isolated paths, then use the counting function fp, to find nodes

aci with exactly one extension such that a has > 2 extensions.

For the decidable class with a non-complemented subclass, the argument of the
Splitting Theorem provides the desired nodes. If P is 1-decidable and has finitely
many isolated paths, then these can be removed to leave a perfect, 1-decidable 11 class

PO. For the perfect class P0, every node a E Ext(Po) has an extension r such that
both -'0 and T-1I are in Ext(Po). Let S = {f E Ext(Po): r0O E Ext(Po) A T'I E

Ext(Po)}. Choose any a E S and let ao = a-0. Now choose any extension r E S of
-71 and let al = r-. Continue in this fashion choosing an extension r S of a-l
and letting On+l = r0.
The fact that Turing degree is not invariant in was proved by D. Martin, using
two maximal sets A and B such that A =T 0', and B theorem:
Theorem 3.2.17 (Soare [47])

Given any two maximal c.e. sets A and B there is an automorphism 4A of E such
that 4(A) = B.
Theorem 3.2.18 Let P be the Ow-class. Given any two maximal c.e sets Wa and Wb
there is an automorphism TI of Cn(P) that: maps Qa to Qb.
It follows from Theorem 2.2.13.

Theorem 3.2.19 Let P be the Ow-class. There is an isomorphism of Cn(P) which
does not preserve the Downey degree.

We observe that the isomorphism of theorem 2.13 preserves degree in sense that
the Turing degree of Qe is the Turing degree of We. Now there exist two maximal

c.e. sets of different degrees, as discussed above.
Lemma 3.2.20 Augmentation Lemma:
Let P = [T] be a non decidable H' class, for each finite a E T \ Ext(T), there

exists a II? class Q such that P C Q, and Q = P U I(acr).
In case of non clopen H' class P, more can be said. In fact, we can exhaust all
such dead-ends, and create h huge IH' superclass for P. For reasons that will become
clear later, we call the following the basis lemma;

Lemma 3.2.21 Basis Lemma:
For every non-clopen II class P, with P = [T], there exists a computable sequence
Op = {TO, TI,...} of pair wise incompatible nodes such that 2W \ P = Un I(n).

Once again use the methods of the Splitting theorem. In this case, we observe
that P is a non-computable subclass of the decidable class 2-.
We can also define an embedding of in Vp'.
Theorem 3.2.22 Let P be a non clopen IIH class such that P = [T] for some com-

putable tree T in 2w. There exists an embedding Cp:

i) Cp : E -, r c n.
ii) (VA E VB E ')[A C B =: Cp(B) C Cp(A)].
iii) (VA C w)[Cp(A) E DEC => A E REC].

and if P E DEC and A is computable then Cp(A) E DEC

Since P is not clopen, we can express 2w \ P as a disjoint union, Un eI(Tn). By
lemma 2.20, for each A C w we define Cp(A) := P U (UOA I(u,)). Note that with
this definition 2- \ Cp(A) = UEA I(rn), and ii) is trivial.
For iii) let us assume Cp(A) is a decidable HI class with Cp(A) = [S] for some
computable tree S. To show A is computable, we reduce A to Ext(T), that is n E A
if and only if r,,n Ext(T).
When P itself is decidable, and A is computable, then for any node r, r E
Ext(Cp(A)) if and only if r E Ext(P) or (3n)(oan -< r A n A). This shows that
Ext(Cp(A)) is c.e. and therefore computable, since Ext(Q) is always co-c.e. for any
110 class Q.
Remark It may be observed that for a non-computable c.e. set A, Cp(A) is not
necessarily a decidable 11' class.
Strong homogeneity for n fails at nodes with a thin IIH class. If A is an infinite
ce set which is the range of some computable function f, then the map We f[We]
is an isomorphism from onto the principal ideal generated by A, (A). This proves
the downward homogeneity of , which makes principal ideals of trivial. In contrast
with this, we observe that: the principal filters of n are not trivial.
Theorem 3.2.23 Failure of the Downward homogeneity for Cr.
Let P be a thin 1Ho class, then Cn(P) is a Boolean algebra. But n is not
This may be compared with the Failure of Upward homogeneity for . If A is
hyperhypersimple then the principal ideal &(A) generated by it is not elementar-
ily equivalent to . The only principal filters whose isomorphism type can be less
complicated than E itself are just those generated by hyperhypersimple sets.


Theorem 3.2.24 If A is nonhyperhypersimple c.e. set, then is embedded in the
principal filter (A) generated by A.
Let {W/(x)}xEW be a disjoint weak array intersecting A. Then the map We
A U (UTEWe Wf(.)) is a one to one map from into (A) which preserves inclusion.


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Farzan Riazati was born in Tehran, IRAN. He received a bachelor of science degree

in applied mathematics and computer science in May 1988, and a master's degree in

mathematics in 1990. For more studies in Logic and the foundations of mathematics

he traveled to the United States. After some graduate studies at the University of

Colorado at Boulder, and Pennsylvania, he transferred to the University of Florida.

He is interested in computability theory in general; and the H' classes, and the

problem of the automorphisms of the lattice of II classes in particular. Many of his

mathematical interests continue to be challenging personal ventures. Travel, and the

challenge of adjusting to new environments are his non-mathematical hobbies. He is

especially fascinated with his new homeland, the United States of America.

I certify that I have read this study and that in my opinion it conforms to accept-
able standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Professo uglasCenz, Chairman
Professor of Mathematics

I certify that I have read this study and that in my opinion it conforms to accept-
able standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

"Scn4-2. 4&~-t^4___
Louis S. Block
Professor of Mathematics

I certify that I have read this study and that in my opinion it conforms to accept-
able standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Williai /Mitchell
Professor of Mathematics

I certify that I have read this study and that in my opinion it conforms to accept-
able standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Beverly A. inders
Associate Professor of Computer Science

I certify that I have read this study and that in my opinion it conforms to accept-
able standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Rick L. Smith
Associate Professor of Mathematics

This dissertation was submitted to the Graduate Faculty of the Department of
Mathematics and to the Graduate School and was accepted as partial fulfillment of
the requirements for the degree of Doctor of Philosophy.

August 2001 ------------
Winfred M. Philips
Dean, Graduate School


Farzan Riazati
Department of Mathematics
Chair: Professor Douglas Cenzer
Degree: Doctor of Philosophy
Graduation Date: August 2001

Pi-0-1 Classes are effectively closed sets. They play a fundamental role in com-
putability theory, along with the computably enumerable sets. Effectively closed sets

can represent the sets of solutions to many important mathematical problems. For

example, the set of zeros of a computable real function, the set of ideals of a com-

putable ring, and the set of 4-colorings of a computable graph, are all effectively

closed sets. Analysis of the complexity of effectively closed sets and their members

can therefore illuminate the difficulty of solving these problems.

The lattice of computably enumerable (c.e.) sets under the inclusion (or subset)

order, has been a major focus of effort in computability theory for the past 50 years.

The work in this thesis concerns the lattice of fI? classes (modulo finite difference),

compared and contrasted with the lattice of c.e. sets. The notion of a minimal

extension Q of a class P is defined to mean that there is no class strictly between

P and Q. Previously only trivial examples were known, but here are given quite

general conditions under which P has a minimal extension. Recently initial segments

of the lattice (that is, subsets of a given set) have been studied. It was shown, in
contrast to the lattice of c.e. sets, that a finite lattice can be realized which is not a

Boolean algebra; in particular, any finite ordinal can be realized. This thesis improves
these results by constructing a 1? class P such that the family of subclasses of P is

isomorphic to the smallest infinite ordinal (omega). Also studied are definability of
various properties (such as finiteness) and invariance under automorphism.

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