PARTITION PROPERTIES AND HALPERN-LAUCHLI THEOREM ON THE
C,mi FORCING
By
YUAN-CHYUAN SHEUf
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2005
ACKNOWLEDGEMENTS
I would like to thank my advisor, Jindrich Zapletal, for his patience and almost
infinite discussion with me on the problems I was trying to solve. Without him, I
simply could not have got this far.
I would also like to thank the other members of my supervisory committee:
Dr. Jean A. Larson, Dr. William Mitchell, Dr. Rick L. Smith, and Dr. Greg Ray. I
have learned a lot from them either through taking classes taught by them, personal
conversations, or hearing them speak in seminars and conferences.
I am very grateful to the Department of Mathematics for supporting me with
a teaching assistantship during my years as a graduate student. I also would like
to thank the College of Liberal Arts and Science for supporting me with a Keene
Dissertation Fellowship in the spring 2005 term.
Finally, I must acknowledge the contribution of my family and all my friends
in the Department of Mathematics for their caring and support.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS .........
ABSTRACT .................
CHAPTERS
1 INTRODUCTION ..........
2 PRELIMINARIES ..........
2.1 Basic Definitions ........
2.2 The Property of Baire .....
2.3 The space 2 ..........
2.4 The Cmin Trees.........
2.5 Determinacy ..........
3 BLASS PARTITION THEOREM .
3.1 Blass Theorem on the Perfect S
3.2 Partition Theorem on the Cmin
4 CANONICAL PARTITION THEOR
4.1 Perfect Tree Case .......
4.2 Cmin Tree Case .........
5 HALPERN-LAUCHLI THEOREM
5.1 Introduction ..........
5.2 C,in Tree Case .........
6 OTHER FORCING NOTIONS ...
6.1 The E0 Trees ..........
6.2 The Silver Forcing .......
6.3 The Packing Measures .....
REFERENCES ................
BIOGRAPHICAL SKETCH .........
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. . iv
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. . 6
S. .. .
. .. 7
ets .......8..... .... 8
Tress ... ... .... ... 10
EM .. .. .. 17
. .. 17
. . 18
. . 24
. 24
24
. 25
. 33
. 33
. 34
.. . 36
. 38
. 39
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
PARTITION PROPERTIES AND HALPERN-LAUCHLI THEOREM ON THE
Cmn FORCING
By
Yuan-Chyuan Sheu
May 2005
Chairman: Jindrich Zapletal
Major Department: Mathematics
In this dissertation, we focus on a special breed of forcing notion called Cmin
forcing, which is forcing equivalent to the form B(R) \ I for some Cmin-ideal I.
We define a special perfect subsets of the space 2", called Cmin trees. We study
the partition properties and Halpern-Liuchli theorems on these trees. We obtained
three principal theorems which will be our discussion Chapters 3,4 and 5 respectively.
We also show that the some other forcing notions do not have similar partition
properties. This is in Chapter 6.
CHAPTER 1
INTRODUCTION
This dissertation is based upon the deep combinatorial theorem of Halpern-
Liuchli [6] on the perfect trees which Halpern and Liuchli developed in the 1960's.
The Halpern-Luchli Theorem is obtained as a byproduct of the proof of Con(ZF +
-,AC + BP), where BP is the statement that there exists a prime ideal in every
Boolean algebra, a consequence of the Axiom of Choice equivalent to Compactness
Theorem for the First-Order Predicate Calculus. It has been noticed since then that
the lemma of the proof might be of independent interest.
One of the main applications of the Halpern-Liuchli Theorem is the proof of
the well known Blass Theorem [1] on perfect trees, which states
T -- [S]
Let a Cmin tree [14] be a nonempty binary tree T C 2<" such that every node
t E T can be extended both into a splitnode of even length and into a splitnode
of odd length. The study of the Cmin trees was originated from Geschke et al. [4],
where the authors defined the Cmin function (see Chapter 2 for definition). The Cmin
a-ideal I generated by the C,i, homogeneous sets is nontrivial and the subsets of 2"
that contain all the branches of a Cmin tree are the typical I-positive sets. The Cin
trees as a forcing notion ordered by inclusion is proper.
We have shown that for the Cmin trees we have a similar but different
T -+ [Si],2,-1,(_-1)!
This is contained in Chapter 3.
In 1930 Ramsey [11] proved the famous pigeon-hole principle for finite sets. A
new situation arises if partitions into an arbitrary number of classes are considered.
For this case, Erd6s and Rado [3] proved the so-called canonical version of Ramsey
Theorem.
Based on the Cmin version of Blass Theorem, we also proved the canonical
theorem on Borel equivalence relations on Cin trees. This means that, fixing a
shape and parity pattern, there exists a finite set G of equivalence relations on [2"]"
such that for any Borel equivalence relation f on [2W"] there is an equivalence relation
g E G and a Cmin tree T C 2" such that f t [T]" = g [ [T]". This is in Chapter 4.
We obtained a Cmin tree version of the Halpern-Liuchli Theorem. In a 1984
paper, Richard Laver [8] generalized the existing Halpern-Liiuchli Theorem to the
infinite product of perfect trees. From there, equivalent results on Hilbert cubes and
selective ultrafilters can be derived. We have developed the Halpern-Liiuchli Theorem
on the infinite product of Cmin trees and we have the equivalent results on Hilbert
cubes and selective ultrafilters as well. This is contained in Chapter 5.
We have also shown that the Cmin trees are special by constructing counterex-
amples that demonstrate that no such results are possible for a variety of other kinds
of forcing notions such as Eo Forcing, Silver Forcing and the forcing notion associated
with the Packing Measure. This is the main content of Chapter 6.
CHAPTER 2
PRELIMINARIES
2.1 Basic Definitions
A relation is a set R all of whose elements are ordered pairs. f is a function
iff f is a relation and
Vx E dom(f) 3!y E ran(f) ((x,y) E f).
f : A B means f is a function, A = dom(f), and ran(f) C B. If f : A -+ B
and x E R, the set of all real numbers, f(x) is the unique y such that (x, y) E f;
ifC C A, f r C = f n C x B is the restriction of f to C, and f"C = ran(f [ C)
={ (f() : x e C}.
A total ordering is a pair (A, R) such that R totally orders A; that is, A is a
set, R is a relation, R is transitive on A:
Vx, y, z E A (xRy and yRz xRz),
trichotomy holds:
Vx, y E A(x = y or xRy or yRx),
and R is irreflexive:
Vx E A it is not the case that xRx.
As usual, we write xRy for (x, y) E R.
We say that R well-orders A, or (A, R) is a well-ordering iff (A, R) is a total
ordering and every non-empty subset of A has an R-least element.
Definition 2.1.1. (a) A partial order is a pair (P, <)such that P I 0 and < is a
relation on P which is transitive and reflexive (Vp E P(p < p)). p < q is read p
extends q". Elements of P are called conditions.
(b) (P, <) is a partial order in the strict sense iff it in addition satisfies
Vp, q(p < q and q < p -+ p = q). In that case, define p < q iff p q and p # q.
Definition 2.1.2. An equivalence relation on a set X is a binary relation ~, which
is reflexive, symmetric, and transitive: For all x,y, z E X, we have that x ~ x,
x y -+ y x, if x ~ y andy z then x z.
Definition 2.1.3. A tree is a partial order in the strict sense (T, <), such that for
each x E T, {y T : y < x} is well-ordered by <.
The set of all finite sequence of O's and l's, 2<1 is a tree with any s, t E 2',
s < t iff s C t. For any T C 2< let [T] be the set of all branches through the tree
T. Any TC 2< is a tree ifVt T and s = t [ n for some n, then sET.
Definition 2.1.4. A skew tree is a tree so that on each level there is at most one
branching node.
For distinct a, p E [T], let d(a, 3) be the level of the highest common node
of the paths a and i through T. Here we assume for any n-set it is the case that
the elements are listed in lexicographic order, and we may assume that the trees are
skew.
Definition 2.1.5. By the shape of an n-element set {ao, .., an-1} C [T], we mean
the linear ordering of {1, ..., n 1} given by i -< j -+ d(ai-1, ai) is in a lower level
than d(aoj_, aj).
Definition 2.1.6. By the parity pattern p, of an n-element set {ao, ..., an-1} E [T]n,
we mean p E 2"-' given by p(i) = 0 if the i-th (counting from 0) lowest splitnode of
the n-set is on the even level.
Definition 2.1.7. Let T = (Ti : i < d) be a sequence of trees. Define (&T= i
to be the set of all n-tuples X from the product of {Ti : i < d} such that IX(i)I =
IX(j)| for any i,j < d; for A C w, let (A T be the set of all n-tuples Y from the
product of {Ti : i < d} such that IY(i)l = IY(j)l E A for any i,j < d.
Definition 2.1.8. An algebra of sets is a collection S of subsets of a given set S
such that
(i) S S,
(ii) if X S and YES then X U Y E S,
(iii) if X E S then S \X E S.
A a-algebra is additionally closed under countable unions:
(iv) IfXn E S for all n, then U'oXn E S.
For any collection X of subsets of S there is a smallest a-algebra S such that
S D X; namely the intersection of all a-algebras S of subsets of S for which X C S.
Definition 2.1.9. A set of reals B is Borel if it belongs to the smallest a-algebra B
of sets of reals that contains all open sets.
2.2 The Property of Baire
Let us call a set A C 2" nowhere dense if the complement of A contains a
dense open set. Note that A is nowhere dense just in case that for every non empty
open set G, there is a nonempty open set H C G such that A n H = 0. A set A is
nowhere dense if and only if its closure A is nowhere dense.
A set A C 2" is meager if A is the union of countably many nowhere dense
sets.
Definition 2.2.1. A set A has the Baire property if there exists an open set G such
that AAG = (A \ G) U (G \ A) is meager.
Clearly, every meager set has the Baire property. Note that if G is open, then
G \ G is nowhere dense. Hence if AAG is meager then (2W \ A)A(2W \ G) = AAG is
meager, and it follows that the complement of a set with the Baire property also has
the Baire property. It is also easy to see that the union of countably many sets with
the Baire property has the Baire property and we have:
Lemma 2.2.2. The sets having the Baire property form a a-algebra; hence every
Borel set has the Baire property.
2.3 The space 2W
Let w be the set of all natural numbers. The space 2W is the set of all infinite
sequences of numbers 0 or 1, (an : n E w), with the following topology: For every
finite sequence s = (ak : k < n), let
O(s) = {f 2 : s C f} = {(ck : k E w) : (Vk
The sets O(s) form a basis for the topology of 2". Note that each O(s) is also closed.
The space 2" is separable and is metrizable: consider the metric
1
d(f, g)= 2-
where n is the least number such that f(n) : g(n). The countable set of all eventually
constant sequences is dense in 2". This separable metric space is complete, as every
Cauchy sequence converges.
2.4 The C,,, Trees
Definition 2.4.1. Let Cmin : [2"]2 -+ 2 be the mapping defined by
Cmin(x, y) = A(x, y) mod 2,
where A(x, y) is the least number n such that x(n) # y(n), and let I, the Cmin-ideal
be the a-ideal a-generated by the Cmin homogeneous sets.
The above definition can be found in Geschke et al. [4]. It is not difficult to
verify that Cmin homogeneous sets must be meager. It is so because if A is Cmin-
homogeneous, say Vx, y E A Cmin(x, y) = 0, then all the split nodes of A are on even
levels.
Now, it is clear that A is meager. And hence the ideal I is non-trivial.
Definition 2.4.2. A Cmin tree is a nonempty tree T C 2
t E T can be extended both into a splitnode of even length and into a splitnode of odd
length.
The following theorem can be found in Shelah and Zapletal [12].
Theorem 2.4.3. Every analytic subset of 2W is either in the ideal I or it contains
all branches of some Cmin tree. Under AD (see below) this extends to all subsets of
2".
2.5 Determinacy
With each subset A of 2" we associate the following game GA, played by two
players I and II. First I chooses a number ao E {0, 1}, then II chooses a number
b0 E {0, 1}, then I chooses al, then II chooses bl and so on. The game ends after
w steps; if the resulting sequence (ao, bl, al, b, ...) is in A, then I wins; otherwise II
wins.
A strategy (for I or II) is a rule that tells the player what move to make
depending on the previous moves of both players. A strategy is a winning strategy
if the player who follows it always wins. The game GA is determined if one of the
players has a winning strategy.
The Axiom of Determinacy (AD) states that for every A C 2W, the game GA
is determined.
CHAPTER 3
BLASS PARTITION THEOREM
3.1 Blass Theorem on the Perfect Sets
Let 2W be ordered lexicographically. For X C 2", let [X]n be the set of n-
element subsets of X. When we describe a finite subset of 2" by listing its elements,
we always assume that they are listed in increasing order. Thus [2"]n is identified
with a subset of the product space (2W"), from which it inherits its topology. A subset
A of 2W is perfect if it is nonempty and closed and has no isolated points. This is
equivalent to saying that A = [T], where T is a tree such that for every t E T there
exist so, sl D t, both in T, that are incomparable, i.e., neither so D si nor sl D so.
A. Blass [1] proved the following
Theorem 3.1.1. Let P be a perfect subset of 2W and let [P]n be partitioned into
a finite number of open pieces. Then there is a perfect set Q C P such that [Q]n
intersects at most (n 1)! of the pieces.
It was pointed out in Blass [1], that the theorem remains true if 2W is replaced
by the real line R with its usual topology and order. To see this, it suffices to
observe that every perfect subset of R has a subset homeomorphic to 2" via an order-
preserving map and that any one-to-one continuous image in R of a perfect subset
of 2" is perfect in R Second, the hypothesis that the pieces of the partition are
open can be greatly relaxed. Mycielski [9, 10] has shown that any meager set or
any set of measure zero in [R]n is disjoint from [P]n for some perfect P C R For
the meager case, he obtains the same result with R replaced by any complete metric
space X without isolated points. It follows that, if [R]" is partitioned into finitely
many pieces that have the Baire property, then their intersections with [P]" are open
for some perfect P. Similarly, if the pieces are Lebesgue measurable, they become G6
sets when restricted to [P']" for suitable perfect tree P'; since G6 sets have the Baire
property, we can apply the preceding sentence, with P' as X, to get a perfect P C P'
such that the pieces intersected with [P]n are open in [P]n. Thus the theorem, as
extended by the first remark as above, implies the following
Corollary 3.1.2. If [R]" is partitioned into finitely many pieces that all have the
Baire property or are all measurable, then there is a perfect set Q C R such that [Q]n
meets at most (n 1)! of the pieces.
Some hypothesis about the pieces is necessary, however, for Galvin and Shelah
[5] have shown that there is a partition of 2' into infinitely many pieces such that,
for any Q C 2W of the cardinality of the continuum, [Q]2 intersects all the pieces.
As mentioned in Blass [1], we may assume that the all the perfect trees are
skew (Definition 2.1.4.). Since -< (see Definition 2.1.5.) tells us in what order the
paths ai split apart as we proceed up the tree and there are (n 1)! linear orderings
-< of {1, ..., n 1} and all of them are obviously realized as shapes within any perfect
subset of [T], we see that the (n 1)! in the theorem is optimal.
We shall need a few definitions: Let T= (To,..., Tr-1) be a skew r-tuple of
perfect trees, which means not just each tree Ti is skew but also that no two distinct
T,'s have splitting nodes at the same level. Let n= (no, ..., nr-) be a r-tuple of
positive integers with sum n. By an n-set in T we mean an r-tuple a whose ith
entry oa is an ni-element subset of (Ti). The shape -< of such an n-set a is the linear
ordering of the pairs (i,j), with 0 < i < r and 1 < j < n,, given by the levels of
d(aj_1, ai,j), where ai, is the j-th element of cr in lexicographic order. The above
theorem is a special case when r = 1 of the more generalized theorem.
Theorem 3.1.3. (Polarized Theorem) Let r, T, n, n be as above and let -< be any
shape of n-sets. Let the collection of all n-sets in T be partitioned into finitely many
open pieces. Then there exist perfect trees T' C Ti such that all n-sets in T' with
shape -< lie in the same piece of the partition.
3.2 Partition Theorem on the C,i. Tress
Lemma 3.2.1. Every Cmin-tree has a Cmin skew subtree.
Proof: Given any Cmin-tree T. To obtain a skew subtree, we must make sure
that at each level, there is at most one branching node. Also, we have to make sure
that for each node t E T, there are branching extensions to, tl E T of t with Ito| even
and Itl odd. We will construct the Cmin skew subtree T' of T by induction on the
number of branching nodes below the nodes we are currently looking at, each stage
make sure the above two requirements are satisfied:
Suppose we have done up to stage n. At stage n + 1, look at all the maximal
(meaning there is no node extends it so far in the construction) nodes constructed so
far and make sure each of them has two extensions of branching nodes, one of them
has even length, one of them has odd length, and so that at each level there is at
most one branching node. Now it is easy to see that T' is a Cmin subtree of T.
The parity pattern p for an n-set r in a sequence of trees T is defined by
p (i) = 0 iff the i-th lowest splitnode in aj is on the even level.
Theorem 3.2.2. (Polarized Theorem for the Cmin trees) Let T=< To, T1,...,Tr-1 >
be a set of Cm,, trees, n=< no,...,n,-1 >, and let -< be any shape of n-sets, p be
a parity pattern of n-sets. Let the collection of all n-sets in T be partitioned into
finitely many open pieces. Then there exist Cmin trees Ti C Ti such that all n-sets in
T' with shape -< and parity pattern p lie in the same piece of the partition.
It easily follows from the theorem as a special case when r = 1, we also have
the following corollary.
Corollary 3.2.3. Let T be a Ci-tree, and let [T]" be partitioned into a finite number
of open( in [T]n) pieces. Then there is a Cmin-tree S C T such that [S]n intersects at
most 2"-' x (n 1)! of the pieces.
Note that the 2n-1 x (n 1)! in the conclusion of the corollary is optimal since
there are (n 1)! many shapes and 2"-' many parity patterns for the sets B E [T]n,
where T is a Cmin tree.
Before we prove the theorem, let me point out that it is easy to show (using
a typical fusion argument; see the theorem below) that any meager set in [2"]n is
disjoint from some [P]n for some Cmin-tree P. Hence we can generalize the theorem
so that the partition is Baire. The proof follows the original proof of Blass on perfect
sets.
Theorem 3.2.4. Given a Cmin-tree T, a natural number m and a meager set A C
[T]m, there exists a Cmin subtree P of T, such that [P]m n A = 0.
Proof: Before we prove the theorem, we need some definitions. The proof
uses the technique of fusion. Let p be a Cnin-tree. A node s E p is a splitting node if
both s^O and s^l are both in p; a splitting node s is an n-th splitting node if there
are exactly n splitting nodes t such that t C s. For each n > 1, let p <: q if and only
if p < q and every n-th splitting node of q is an n-th splitting node of p.
A fusion sequence is a sequence of conditions {p : n E w} such that pn
Pn+-. It is clear that if {p, : n E w} is a fusion sequence then nnEwPn is a Cm,i-tree.
If s is a node in p, let p [ s denote the tree {t p : t C s or t D s}. If B
is a set of incompatible nodes of p and for each s E B, q, is a Cmin-tree such that
q, C p r s then the amalgamation of {q, : s E B} into p is the tree
{t E p : if t D s for some s E B then t E q,}.
Now we start the proof of the theorem by constructing a fusion sequence { : n E w}
of Cmin-trees by induction on the level of the split nodes n: We may assume that
A = UiEAi, where each Ai is closed nowhere dense.
Let d be large enough such that 2d > m. We will begin our induction stage 0 on
the set of d-th branching nodes {so,..., S2d-}1) f T. For each si find two nodes s~O^'t
and s 1^t! such that each |tI is even and Itl| is odd and for any n-tuple (xo,..., xm-1)
in [T]m with each xj in a different Cmin tree T [ s-a^-t, where a, b {0, 1}, we have
that (xo, ..., Xm-1) Ao. This can be done using the fact that Ao is closed nowhere
dense. Let Po be the amalgamation of {T [ s a^-t : a, b = 0,1}. It is clear that Po is
Cmin. Suppose now we have obtained Po, ..., Pn at stage n. At stage n + 1 repeat the
process on the d + n + 1-th branching nodes of Pn such that all the m-tuples selected
as above from different Cmin subtrees of Pn avoid the set {Ao,..., An+i} and obtain
Pn+1.
Let P = n,,,P,. Now we have that [P]" n A = 0.
Proof of theorem 3.2.2.: By remove node s from T," we mean that T is
to be replaced by its largest subtree not containing s; kill the branching node s of
T" means to remove( in the sense just explained) one of the two immediate successors
of s. The choice of which successor to remove is arbitrary except when a specific node
t above s is to be retained"; then the immediate successor of s that is not below t
is to be removed.
We proceed by downwards induction on r, with n fixed. Since each ni is
required to be positive, the highest possible value of r is n, and if r = n each ni = 1.
An n-set from T is then just an n-tuple a of paths ai through Ti. Fix such an
n-set a. Since the partition is open, all n-sets sufficiently close to a lie in the same
partition class. Thus, for sufficiently long finite initial segments si of as, the trees
Ti = Ti r si satisfy the conclusion of the theorem.
We turn our attention to the nontrivial case r < n and assume that the
polarized theorem for r + 1. Without loss, we may assume that for any n-set o the
lowest-level branching point f(a) is in To. In each of the remaining trees Ti(1 < i <
r), all of the paths aij pass through the same node s, at the level of f(o). We call
the r-tuple (f(ar), sl,..., sr-i) the signature of the n-set a.
Lemma 3.2.5. There are Cmin Ti* C Ti so that any shape -< and parity pattern p,
all the n-sets having the same signature lie in the same partition class.
Proof: Each tree Ti* will be obtained as the intersection of an inductively
defined decreasing sequence of Cmin subtrees of Ti. At each stage n, build Cmin
subtrees Ti" of Ti and select some number ,n so that any s E 2<1 that is in the tree
T77 and below level In will be in the trees Tim for all m > n. In the end T* = nn<,T"n.
To begin the induction, set To = Ti, l=0.
Suppose that, at a later stage n, we have obtained Cmi, subtree Tin and some
number ,I < w. Choose a level 1, so high that in each Ti(i # 0) every node s at level
In has at least two successors tl, t2 that are branching nodes and such that It | is odd
and It21 is even; this can be done since each Ti" is Cmin. Kill all the branching nodes
of Ton between level In and 1, inclusive and let 1,,n = In.
Next, choose a branching node f of To" above level l~ say at 1~ and kill all the
branching nodes between l, and l 1 inclusive, retaining f, and let ln+i = lN.
We now seek to make sure that all n-sets with shape -< and parity pattern p
with f as the first branching node have their partition classes determined by their
signatures. Enumerate all the possible signatures (f,sl, ...,sk) that begin with f;
there are only finitely many, say m, of them. Consider each such signature in turn.
Construct Cmin subtrees T, ...,T"'m of each Ti by a finite induction as follows:
Suppose we have constructed up to some TI'j at some stage j, for stage j + 1,
consider:
The n-set r in T"'J with shape -< and parity pattern p and the j + 1-th
signature (f, sl, ..., Sr-) yields, by splitting the first component, an n*-set
c* = ({ao,..., oq-1, o,q, ,---, O,no-l a1, ', r-1),
which is in
Tn"j, = (To'j [ fr O, T~"j f'l,T'n, r si,...,TrTJ r sr-1),
where n* is the (r+ 1)-tuple (q, no q, n, ..., nr-i). The shape -<* and parity pattern
p* of ar* is uniquely determined by -< and p, and r* determines o. Partition the
n*-sets in T"n,'* with shape -<* and parity pattern p* by putting two such o*'s in the
same piece if and only if the corresponding o's are in the same piece of the original
partition. Since a is a continuous function of or*, this is an open partition of n*-sets,
and we can apply the induction hypothesis to find Cmin subtrees of the Tin7'*'s such
that all their n*-sets with shape -<* and parity pattern p* lie in the same piece of
partition class. Prune the trees Ti', correspondingly and call them Ti'+' so that the
new
Tnj+l [ f^O, To'n+1 f- 1, Tn,7j+l r sl, s ., Tn+.. 1 [ S-1
are the subtrees of the T "~'*'s given by the induction hypothesis.
Let T+1 = nj
Now, let T* = n
subtree. By carefully selecting the f's, To can be arranged to be C,in as well. We
may assume this T* is good for each fixed shape and parity pattern by repeating the
process finitely many times. This completes the proof.
We now seek to eliminate the dependence of the partition class on the signature
for the n-sets with fixed shape -< and parity pattern p, by reducing the trees still
further.
Without loss, let us assume that po(0) = 0; i.e., the lowest split node of oo is
on the even level. To each r-tuple s with si e Ti* we associate a signature s of T*
as follows. Let gs be the highest even level branching node C so in T* (It will not
matter how we define if so is below the lowest even level branching node of To.)
For i 0 0, let si be the predecessor or the lexicographically first successor of si at the
level of go. Clearly, s = (so, ...-r 1) is a signature of T* for the n-sets with shape -<
and parity p.
We apply the Halpern-Liuchli partition theorem [6] to the partition of T* x
... x Tr- = T* sending two r-tuples to the same partition class determined by the
last lemma with shape -<, parity pattern p if their signatures are in the same partition
class. It asserts that there is a natural number h such that one of the partition classes,
say c, contains (h, k)-matrices for all natural numbers k. This means that, for each
k, there is a sequence x of nodes xi at level h in T7* and there is a sequence A of
subsets Ai C T* such that
(1) every successor of xi at level h + k in Ti* is below a node in Ai, and
(2) all r-tuples s E f A lie in the partition class c.
Fix such an h. There are only finitely many tuples x as above, so the same
x works for arbitrarily large k; hence we may assume it is for all k. Fix such an x.
The process of pruning the trees T* begins with setting Si = T* [ xz. To simplify
notation, we assume that h = 0; the general case involves adding h to every level
mentioned in the sequel.
Now proceed by induction, at each stage n build Ci, subtrees T*'"n of T*
and select some number I, so that any s E 2
level l, will be in the T7'" for all the m > n. In the end T[ = nn<,Ti'". To begin
the induction, set T'i* = Ti* and lo = 0.
Suppose that at a later stage n, we have obtained Cmin subtrees Ti*'n and some
number In < w. In stage n + 1, choose 1, so large that, in each T*'"(i $ 0), every
node s at level In has at least two branching nodes to, tl above s and below level I'n
so that Ito| is even and Iti| is odd. Next, choose an even level branching node f in
To'' above level 1 say at level 1~ > l'. By Halpern-Liiuchli, find Ai C Ti so that
(1) Ai dominates all nodes of level I" in T*'", and
(2) HAC c.
By (1), find so E Ao above f. Then s0 is above or equal to f, so its level 1* is
greater or equal to l'. Kill all the even level branching nodes of To*n from level In to
level 1* 1 inclusive retaining so. As before, this ensures that go is the unique even
level branching node of To*' between levels In and 1 inclusive. For any i : 0 and
any si E Ai, let be, as before, the predecessor or lexicographically first successor
of si at level 1*. Then Bi = {ilsis E Ai} dominates level l' of T*'" because 1* > l'
and (1) holds. Furthermore, by (2) and the definition of the partition for r-tuples
other than signatures, {go} x fi40 Bi is included in class c of the original partition
of signatures. For i : 0, remove all nodes of Ti*'" that are not comparable with any
node in Bi, and let the resulting tree be T*'"+1 and let ln+1 = 1* + 1. All nodes at
level 1 in T,n+l(i 0) belong to Bi, so all signatures that start with s0 are in class
C.
Now let T.' = n,< ,Tj". It is clear that for each i $ 0, we have that T[ is Cmin.
To can be arranged to contain a Cmin tree as well by carefully choosing the f's.
CHAPTER 4
CANONICAL PARTITION THEOREM
4.1 Perfect Tree Case
In 1930 Ramsey [11] proved his famous pigeon-hole principle for finite sets:
Theorem 4.1.1. Let n, r be positive integers and let X be a countable infinite set.
Then for every partition A : [X]n --+ {0,...,r 1} of the n-element subsets of X
into r many classes, there exists an infinite subset Y C X such that the restriction
A f [y]n is a constant mapping.
A new situation arises if partitions into an arbitrary number of classes are
considered. For this case, Erd6s and Rado [3] proved the so-called canonical version
of Ramsey theorem:
Theorem 4.1.2. Let n be a positive integer and let X be a countable infinite set,
which is totally ordered. Then for every mapping A : [X]n -+ X there exists an
infinite subset Y C X and a subset I C {0, ..., n 1} such that for all {ao, ..., an-},
{bo,..., bn,_} E [Y]" with ao < ... < an-~ and bo < ... < bn- we have that
A({ao,..., an1}) = A({bo,..., bn-1}) if {ai : i E I} = {b : i E I}.
For many other structures like for example arithmetic progressions, parameter-
words and finite vector spaces, canonical partition results are known.
H. Lefmann [7] considered perfect-tree version of these theorems in the space
2" with the metric
1
d(x, y)= -iff k = min{n: x(n) # y(n)}.
and have the following result:
For any A E [T] A = {ao,..., a,-1} and J C {1,...,n- 1}, let D(A) : J =
{d(a-1, a,) : j E J}, where d(x, y) is the distance between x, y.
Theorem 4.1.3. Let n be a positive integer and let ({1,...,n 1},-<) be a total
order. Further let T C 21 be a perfect subset and let X be a metric space. Then
for every Baire-mapping A : [7T1] -- X there exists a perfect subset P C T and
subsets I C {0,..., n 1} and J C {1,...,n 1} such that for every A,B E [7P]n with
A = {ao,..., aLn-1}<,ex and B = {(o, .., /n-1}
A(A) = A(B) iff {a : i I} = { : i E I} and {d(aj-_,aj) : j J} =
{d(1pj-, [j) : j J}.
4.2 C,,a Tree Case
Lemma 4.2.1. For any Borel equivalence relation ~ on a Cmi, tree T C 2", there
is a Cmin tree P C T so that either x ~ y for all x, y E P or x ~ y iff x = y for all
x, yE P.
Proof:
Let ~ be a Borel relation on T. This induces a mapping A* : [T]2 {0, 1} by
A*({ao, al}) = 0 iff ao ~ a,. Clearly, A* is a Borel mapping. By the Cmin version
of Blass Theorem, there exists a Cmin subtree P C T, either A*([P]2) = i for some
i = 0, 1, or A*(ao, a1) = 0 iff ao, al split on even (odd) level. We have the following
four cases:
(1). If this P is such that A*([P]2) = 0, then obviously we must have that
a0o ~ a for any ao, a1 E P.
(2). It is the case that A*([P]2) = 1, then we have that ao ~ a1 iff ao = a,.
(3). It is the case that A*(ao, al) = 0 iff ao, al split at an even level. This
does not happen, since is a Borel equivalence relation.
(4). It is the case that A*(ao, ai) = 0 iff ao, a1 split at an odd level. Again,
this does not happen since ~ is a Borel equivalence relation.
Let A C 2" be finite with A ={ao, ...,an-1} ordered lexicographically. Let
D(A) ={d(ai_1, ai) : 1 *
of A. Further let I C {0,..., n 1} and J C {1,..., n 1}. Then*
A: I = {ai:i E I}
is the I-subset of A and
D(A) : J = {d(ajl, a) : j E J}
is the J-subset of D(A).
Theorem 4.2.2. Let -< be a total order on {0,..., n 1}, p E 2n-1 and T be a Cmin
tree. Then for any Borel equivalence relation ~ on [T],,, there is an I C {0,..., n-1}
and J C {1,..., n-1} and a Cmin P C T such that for any x, y E [P]",,, the following
are equivalent:
(1) x y.
(2) x: I = y: I and D(x) : J = D(y) : J.
Proof: For n = 1, this is just the lemma.
For the induction step, use the special constructions from [7]. Let T E [T] p
with T = {ao, ..., -1}<,,- Define a mapping f : {0,..., n 1} -- {0,..., n- 1}
where f(i) is the unique i+j with j E {-1, 1} such that ainai-j is an initial segment
of ai n ai+j.
Definition of T + i.
For i < n let T + i E [7T]"1 result from T by adding a new element at E T
to T such that the following is valid:
(i) at -
(ii) aci n at is an initial segment of ai n af(i).
(iii) for j = 0, 1 it holds that if (ai n af(i)) j is an initial segment of at; thus
a+ leaves af(i) in the same direction as a, does.
(iv) T and (T \ {ai}) U {a+} have the same shape and parity pattern.
Definition of T e i.
For i < n let T ei E [T]"n+ result from T by adding a new element a, E T
in the following way:
(i) a?
(ii) ai n af(i) is a proper initial segment of ai n cl
(iii) T and (T \ {aI}) U {af} have the same shape and parity pattern.
Let the resulting sets T + i resp. T e i have shape -
resp. -f and pf. T + i or T E i need not exist for every T [T],p, but certainly
there exist subsets T E [T]n, such that T + i and T e i exists. If they exists, they
are in general not unique, while their order-types, and only these will be of interest
in the following, are independent of the special choice of the added elements.
Now we start with the induction step. Let n > 2 be given and assume that the
theorem is valid for all k < n. Let ~ be a Borel equivalence relation on [T],p. For
every i < n the equivalence relation ~ induces a mapping At : [T]+ {0, 1}
by
A (T) = 0 iff T : ({0,..., n} \ {i}) ~ T : ({0,..., n} \ {i + 1}).
Since the functions At are Borel mappings, by Theorem 3.2.2., there exists a
Cmin subset To C T and for every i < n there exist a number c E {0, 1} such that
A([ ToJ]1,+) = c
Case 1: If this To is such that At([Toj]+ +) = 0 for some i.
For T e [To]" let the resulting set T : ({0, ..., n 1} \ {i}) have shape -<* and
parity pattern p*. Then ~ induces a equivalence relation ~* on [T.n- by
parity pattern p*. Then -, induces a equivalence relation -,* on [*TO-]-.,p by
(To: ({0,...,n 1} \ {i})) ~* (TI: ({0,..., n 1} \ {i})) iffTo ~ Ti.
Restricting to a Cin subtree of To if necessary, we may assume that ~* is Borel and
well-defined.
By induction hypothesis there exists a Cmin subset T1 c To such that there is
some I* C {0, ...,n 2} and J* C {1,..., n 2} such that
n--1
A ~* B iff A : I* = B : I* and D(A) : J* = D(B) : J* for any A, B E [T1]" <..
Now for any To, T, E [7],,, let I C {0,..., n 1} and J C {1,...,n- 1} be such that
for T E [7]",,,, we have that
(T: ({0,...,n-1} \{i})): I*= T:I
and
D(T: ({0,...,n- 1}\{i})): J*=D(T): J.
It follows for To, T1 e [T1]", that
To ~ T1 iff To : I = Ti : I and D(To) : J = D(T1) : J.
Case 2: If To is such that c = 1 for all i:
For i < n the equivalence relation ~ induces a mapping A : [IT]nl, -
{0,1} by
A?(T) = 0 iff (T : ({0, ..., n}) \ {i})) ~ (T : ({0,..., n} \ {i + 1})).
We may assume that each A is continuous. By Theorem 3.2.2., there exists
a C,in subtree T1 C To and for each i < n and di E {0, 1} such that
A? ([T7],) = di.
Consider for every two-element set {To, T1} [[7] <,]2 the union To U T, say To U
Ti = p, of shape -<* and parity pattern p*, let Ko, K1 C {0, ..., p 1} be subsets such
that
(To U T) : Ko =To and (To U T) : K = T.
Let (({1, ...,pi}, -<*, p*), K9, Ki')i
ring triples. For every i < q let Ai : [Ti]p.,. 2 be a mapping defined by
A,(T) = 0 iff (T: Kg) ~ (T: Kg).
Since the functions A, are continuous, by Theorem 3.2.2., we have a ci,
subtree T2 C T7 such that
A r[ [T2]',p = c,where ci E {0, 1} for every i.
For any T E [T2]_,,, let I = {i < n : di = 1}. We claim that for all To, T E
[T2],,, we have that
To ~ Ti iff To : I = T : I and D(To) = D(TI).
For i= 0,1, let Ti = {ai, ..., a~} be given. At first we show the implication
from right to left. So let To : I = T : I and D(To) = D(Ti). If I = {0,.., n 1},
there is nothing to prove. So let I {(0,...,n 1} and take I E {0,...,n 1} \I.
Since dt = 0, we conclude that
{a 0,..., a~ <, ~ }...~ \ {a} U {a}.
Iterating this we get
{ao,...,a o_}<,e, {a : ie o}U {a: j {0,..., n-}\I}
and thus To ~ T1.
Now we prove the implication from left to right. Let To ~ T1 and assume
To : I T : I or D(To) : D(T1). Suppose D(To) $ D(Ti). Let
d = min((D(To) \ D(Ti)) U (D(T1) \ D(To))),
where without loss, d = d(a_, a). Since d is minimal, there is j < 2 such that
a (a9_1 n a9) j is not an initial segment for any / E Ti.
Let a be an initial segment of, say, ao. Choose &i E T2 \ (To U T1) such that
(i) (a n &i) -' j is an initial segment of &,.
(ii) a is not an initial segment of &i.
(iii) To U Ti and ((To U TI) \ {ao}) U {&i} have the same shape and parity
pattern.
Choosing other sets To, T1 if necessary, this is possible. But then we have
To T1 ~ (To \ {ao}) U {da},
contradicting the fact that At([To] +) = 1 for every i.
S(o-
Suppose now D(To) = D(T1) but To : I Ti : I. Then there exists i E I with
a9 $ ail, say af
To U T1 and ((To U TI) \ {al}) U {a} have the same shape and parity pattern. As
before, we get
To ~ T ~ (T7i \ I{as ) U {a},
which contradicts the fact that di = 1. This completes the proof of the theorem.
CHAPTER 5
HALPERN-LAUCHLI THEOREM
5.1 Introduction
In the following we will describe some of the the known facts about the
Halpern-Liuchli Theorem. We will prove a Cmin version of the Halpern-Liiuchli
Theorem and an infinite version of the C,in version of the theorem. Please refer to
Chapter 2 for basic definitions.
Theorem 5.1.1. ( Halpern-Lauchli) Let d be a natural number, let Ti be a perfect
tree of height w for every i < d, T= (Ti : i < d), and let k be a natural number. For
every function f : ( T-+ k there is an A E [w]W and a perfect subtree Ui C Ti for
every i < d such that the function f [ (A U is constant, or in other words, A U
is homogeneous for f.
Let us call the above theorem HLd. In [8], the infinite version of the Halpern-
Lauchli theorem HL, was proved. It has two other equivalent restatements.
Theorem 5.1.2. The following are equivalent:
(i) HL,
(ii) If P, is the partial ordering for adding r side-by-side Sacks reals with
countable supports, then every X C w in V1 contains, or is disjoint from, some
Y E [w]W with Y E V.
(iii) If fi : i < w are continuous functions from the Hilbert cube [0, 1]" into
[0, 1], then there exists non-empty perfect sets Pi C [0, 1](i < w) and A E [w]W such
that, on (i<,, Pi, (fi : i E A) is monotonic.
5.2 C,,i Tree Case
Theorem 5.2.1. ( The Ci, version of the Halpern-Liuchli theorem) Let d, k be
natural numbers, {Ti : i < d} be Cmin trees. Then for any function f : ( Ti --+ k
there is an A E [w]W and Cmin subtrees Ui C Ti such that f [ (A U= c for some
c< k.
The theorem can be easily proved by induction using the following [13]:
Definition 5.2.2. Let T be a tree. A set A C T is a n-dense set if and only if there
is an m > n such that A C T(m), the m-th level ofT, and for every s E T(n) there
exists a t E A such that t D s. Similarly, if To,..., Td- are trees and Ai C Ti(m) for
all i = 0,..., d 1, then the sequence (Ao, ..., Adi) is n-dense set in (To, ...,Td-1) if
and only if for every i < d the set Ai is n-dense in Ti.
Theorem 5.2.3. (Dense set version of Halpern-Liuchli Theorem)
Let d, k be natural numbers, {T, : i < d} be a set of perfect trees. Then for any
function f : Ti -+ k, there is an A E [w]W, such that either one of the following
is true:
(i) Vn E w, there is an n-dense set (Ao,..., Ad-) in (To, ..., Td-) such that
f"Ao x ... x Ad-1 = {0}, or
(ii) There is atE To x ... x Td-1 such that for all n E w there is an n-dense
set (Bo,...,Bd-1) in (To[to], ...Td-[t-1]) such that f"Bo x ... x Bd-I = {1}.
Proof of Cmin-Halpern-Liuchli Theorem: We may work on one dimension
only, because for finite dimension products, the proof is similar. Given any Cmin tree
T, without loss of generality, let us assume that the first case of the dense set version
of the Halpern-Liuchli Theorem occurs. First let n = 0, and find no and Ao that
satisfies the dense set theorem, trim all the nodes that are not comparable with any
node in Ao, i.e cut all the nodes s with no r E Ao such that r C s or r D s. Now find
an m so large that every node t in Ao there are at least two branching nodes tI, t2
above t such that to is even and Iti is odd. Now find nl > m and A1 using Theorem
5.2.3.. Repeat the process infinitely many times, we easily get a tree T' that contains
a Cmin-tree. Eventually, the set {ni : i < w} is the set A required in the Cin version
of the Halpern-Lauchli Theorem.
Theorem 5.2.4. HL, is true for the Cmin trees.
Proof: If d < w and m < k < w, X = (Xi : i < d), X C l
some trees Ti, let Am= {(Bi : i < d) : for each i, Bi C Xi, CardBi = 4 and each
Bi = {t^tos, ttos2,trt^sa,^ t^t 4} for some t E Ti(m) and Itol is odd and It|l is
even }.
Call S a 4-branch subtree of the Cmi tree T if S is the union of four distinct
maximal branches in T and such that t^to and t^tl, where tol is even and Itll is odd,
are the branching nodes above the stem (of the four branches) t.
Lemma 5.2.5. Let d < w and Ti(i < d) be Cmin trees. Let m < w. Then there
is p(m,T= (Ti : i < d)) < w, large enough so that for any G C )i
G n @B / 0 for all BE A,((Ti(p) : i < d)), then there is an m-dense (Yi : i < d),
Y C Ti(p), with @YC G.
Proof: This is a consequence of the dense set version of the Halpern-Laiuchli
Theorem. If for some m and all p a counterexample Gp,m existed, then UpGp,m would
be a subset of (T"(Ti : i < d) not containing the product of an m-dense sequence; by
the dense set version of the Halpern-Liuchli Theorem, there exists t E ) T and for
infinitely many p, a t-p-dense sequence Xp with & Xp nGp,m = 0. Choose p such
that each ti E Ti has two splitting nodes t^to and t"tl above it such that Ito0 is even
and |tll is odd and the splitting nodes are below p; this is a contradiction.
It suffices for the general case to prove the Cmin version of HL, for those
sequences (Ti : i < w) of Cmin trees such that
limit (least level mi where Ti splits) = w.
Let T7 be the collection of such sequences. For T E T,, an n-dense sequence
in T is an (Xi : i < w) such that for some m, each Xi C T (m), (Xo,...,Xn-1) is
n-dense in (To, ..., Tn-1), and Xi 0 0 for i > n. If x E ( T, an x-n-dense sequence
is an n-dense sequence in (Ti [ i : i < w).
Lemma 5.2.6. If T E T, A E [w]O, G C (A T and for each n < w there is an
n-dense X with ( X CG, then there exists Cmin Ti C Ti(i < w) and A' E [A]NO with
(A' T' CG.
Proof: This is clear.
Lemma 5.2.7. If T E T,, A E [w]"O and (A T= Go U G1 then either
(a) for all m there is an m-dense X with & X C Go, or
(b) for some non-zero d < w there exists 4-branch subtrees Si of Ti(i < d),
Cmin subtrees Ti ofTi(i > d), A' E [A]~o, such that
(*) for any infinite set A" E [A']'o and any sequence (Ti" : i > d), with T" a
Cmin subtree of T7, there exists t E 9Al" T", say height t= q, such that for any u
E @i
Proof: Suppose that (a) fails for m, then m will be the d of (b). Let p be
the p(d(Ti : i < d)) of lemma 5.2.5.. Thin down each Ti to T(i < d), such that Ti is
the same as Ti at levels < p, and above level p consists of, for each node t E Ti(p),
a single maximal branch of Ti above t. Let ao,..., a, enumerate Amn((Ti(p) : i < d)).
For each p' > p this induces an enumeration ao(p'),...,al(p') of Am((Ti(p') : i < d)).
Now construct a sequence by induction on 1' < I as far as the construction
can be continued,
Vi. y D ...i D..,
of downwards closed C,mi subtrees of Ti, for each i > d, and a sequence
BO B D ... D B' D ...,
with each Bj an infinite subset of A, so that for each 1' < 1, for any tE B (Vl/ :
i > d) there exists u E (ai, (height t) with u"t E Go.
This thinning process cannot be continued through all +1 stages. Otherwise
there would be tE (A Ti : i < d), level t > p such that for all 1' < 1 there exists
uE (ayl ( height t) with u^t E Go. For u E 1i
if u^t E Go. By definition of p, Go would contain an m-dense sequence; appending
t to it would give an m-dense X in (Ti : i < w) satisfying (a).
Thus we shall get to a 1' < I where Vi', Bil can be chosen but not Vi'+1, B1'+.
Let A' = B1', Ti = Vt'(i > d) and let Si(i < d) be the 4-branch subtrees of Ti(i < d)
determined by ar,+1. These satisfy the property (*).
Lemma 5.2.8. If T E T, and 0" T= Go U G1, then either
(a) for all m there is an m-dense X in T with ( X C Go, or
(b) there are 4-branch subtrees Si of Ti (i < w) and A E [w]'O with A S C G1.
Proof: By Lemma 5.2.7., pick a non-zero do < w, Ao E [w]N, 4-branch
Si C Ti (i < do) and Cmin T C0 Ti(i > do) such that for any B E [A]'o and Cin
Ui C Ti(i > do), (B(Ui : i > do) n Go 0, where we define, for t E Ao(Ui: i > do),
t E Go if and only if for every u E (&'
Also, arrange that there exists te W(To : i > do), tE Go, such that no
T splits before height (t) by thinning down the TO if necessary; let height (t)= ao,
the first member of A. By the assumption that (a) fails, there is m such that no
m-dense sequence X in (To : i > do), with height X E Ao, satisfies ()XnG G 0.
Thus we may carry out the construction again with respect to (TO : i > do), Ao
and G; that is, using lemma 5.2.7, find 4-branch Si C To(do < i < di), Cmmn tree
Ti C To(i > di), A' E [Ao0]N, and GI(te G1 if for every uE <9i
level as t, u^tE GO), with )B(Ui : i > di) n G\ : 0 whenever each Ui is Cmi in Tl
and B E [A']~o, and thin out to pick al > ao as before. Repeat this construction for
infinitely many times, getting (Si : i < w) and A = {an : n < w}; these satisfy the
lemma.
To prove the theorem, we are given "& T= Go U G1, and will construct Cmin
trees Ui(i < w) and a map from (" U into (" T, and show that a homogeneous
(A S (Si a 4-branch subtree of Ui) induces a homogeneous (B T' (T' a Cmin subtree
of T).
We may assume that for each i,
Ti = {s E 2<' : s(j) = 0 Vj < i} = Ki,
for (Ki : i < w) is isomorphic to a sequence of subtrees of (Ti : i < w) restricted to
a common infinite set of levels. Let (B, : n E w) be a disjoint partition of w, with
each Bn infinite. We may assume that each B, has infinitely many odd numbers and
even numbers in it. Now, via some bijection between w and w x w, {Uj : j < w} will
be {Ui,n : i, n < w}, where
Ui,n := {s E 2< : s(j) = 0 whenever j V Bn or j < i}.
Then, if u E0W U, height u =m, define H(u) =t EQ" T, height t =m by
the rule:
S ui,, if i j < m, j E Bn,
t0j) o if j < i.
Then H is a level preserving bijection between (& U and & T. We have now
0W U= Go U G1, where Gj := H-'"Gj. By the last lemma, there exist an infinite
set A C w and 4-branch subtrees Si,n of Ui,(i, n < w) and j < 2 with (&A SC Gj.
Then there are downwards closed subtrees T' of T, with H" &" T', and we have
(A T'C Gj. It is clear that each Ti is Ci. This completes the proof. O
Definition 5.2.9. A nonprincipal ultrafilter U on w is selective if for every sequence
(An : n < w) of elements of U there is A E U such that A \ n c An for every n E A.
Such A is said to be the diagonalization of the sequence (An : n < w).
Lemma 5.2.10. A non principal ultrafilter U on w is selective if for every partition
p : [w]2 -+ 2 there exists i < 2 and A E U such that p"[A]2 = {i}.
Lemma 5.2.11. If U is a selective ultrafilter on w and if A is an analytic subset of
[w]W, then there is an A E U such that [A]W C A or [A]" n A = 0.
Definition 5.2.12. IfU is an ultrafilter on w, let HLd(U) be HLd with the require-
ment that the set A be a member of U.
Theorem 5.2.13. For every d < w and every selective ultrafilter U on w, the Cin
version of HLd(U) is true.
Proof: Let Ti(i < d) be a given sequence of Cmin-trees and let p : 0i
{0, 1} be a given coloring. If d = w, we further assume, without loss of generality,
that if si is the minimal splitting node of Ti for i < w, then the length of s, increases
with i. Let
A = {A E [w] : (Vi < d)(3 Cmin Ui C Ti)(3e < 2)p" (& Ui A = {}}.
i
Then A is clearly an analytic subset of [w]" so by fact 2, there exists A E U
such that either [A]W C A or [A]W n A = 0. Note that by the Cmin version of
Halpern-Lluchli theorem, the second alternative does not happen, so we are done.
Theorem 5.2.14. Any finite product of Cmin forcing preserves the selective ultrafil-
ter, hence does not add any independent sets.
This follows from the next theorem.
Theorem 5.2.15. Let P be the Cmin forcing. Every selective ultrafilter U generates
a selective ultrafilter U* in the forcing extension VPW.
Proof: The only thing left to prove is that
U* = {A C w : 3B E U such that B c A} is selective;
that is, every sequence (An : n < w) of elements of U* has a diagonalization in U*.
But this is easy to show. Consider a fusion sequence P, where s E 2<, i < w, such
that for every n < w and s= (si : i < w) in (2n)W the condition (P, : i < w) forces
that An is equal to some element Bs of U, we may do so since we can without loss
of generality assume that each An is taken from U. Let (Pi : i < w) be the fusion of
this sequence and let B E U be such that B \ n c B, for every n E B and sE (2n)".
This finishes the proof.
We also have the Cmin analogue of the perfect set result on Hilbert cube:
Theorem 5.2.16. For every sequence {fn : n < w} of continuous functions from the
Hilbert cube [0,1] into the interval [0,1], there is a subsequence {f'} and a sequence
{P,} of Cmin subsets of [0, 1] such that {f/} monotonically converges to a continuous
function on the product Po x P1 x P2....
Proof: Let P be the Cin forcing. In the forcing extension of P" look at the
sequence of reals f,((si : i < w)), where (si) is the canonical name for the generic
sequence of Cmin reals. Under the assumption that there is a selective ultrafilter U,
U* is a selective ultrafilter, apply the above fact to the partition, p : [w]2 2 defined
by
p({m, n}) = 0 iff f,((si : i < w)) < fm((s : i < w)),
and get a member A of U* such that fn((si : i < w)) (n E A) is monotonic. Fix
(Pi : i < w) E PW deciding the set A and the fact that the sequence is, say, increasing.
Let
F = {(x : i < w) E fPi : (Vm < n E A)fm((i : i < w)) < fn((x : i < w))}.
i
Notice that F is a closed subset of 1fi<, Pi. Since every nonempty open subset
of this product contains a product of a sequence (Pf : i < w) of Cin subsets which
must also force that the sequence fn((si : i < w)) is increasing, the set F must be
equal to the product 1i<, Pi. Let g : i<, Pi -+ [0,1] be the limit of fn(n E A).
Since g is Baire, by further restricting to the subtrees if necessary, we may assume it
is continuous. So we are done.
Theorem 5.2.17. For every finite d and a sequence {fn} of continuous functions
from the cube [0, 1]d into [0, 1], there is a single Cmin set P C [0, 1] and a subsequence
{fn } which is monotonically convergent on pd.
Proof: The set P will be the result of a fusion sequence P,(s E 2
structed together with a decreasing sequence of set An(n < w) such that for every
n < w and every sequence si of distinct elements of 2" the subsequence {fk :k E An}
when restricted to the product I-i
function. Clearly, there is no problem in constructing this fusion using the previous
result. It should be pointed out that if {kn}n<, is a strictly increasing sequence such
that kn E An for every n < w then {Fk }n<, and the set P satisfies the conclusion of
the theorem.
CHAPTER 6
OTHER FORCING NOTIONS
6.1 The En Trees
Let Eo be the relation on R, such that
for any x, y E R, x ~ y iff x and y differ by a rational.
It is easy to see that E0 is indeed an equivalence relation. When working with
the space 2W, let x ~ y mean that x(n) = y(n) for all but finitely many n.
Let I be the ideal a-generated by the partial Borel Eo-selectors, where a partial
selector is a subset of R which meets each equivalence class in exactly one point. It
is well-known that Borel Eo selectors are meager and it is easy to show that any
member in the E0 ideal I has Lebesgue measure 0, hence Eo is nontrivial. For more
information about the E0 ideal, see [14].
Definition 6.1.1. An Eo-tree is a non empty tree T C 2W which is perfect, that is,
every node in T can be extended into a split node, and homogeneous, i.e., for every
split node t E T there are two longer split nodes so, sl of equal length such that every
node compatible with t is compatible with so or si and {u : s'o E T} = {u : su E T}
Theorem 6.1.2. [14] Every analytic subset of 2W is either in the ideal I or it contains
all branches of some Eo-tree.
The following result is due to Blass in a private conversation:
Theorem 6.1.3. There is a Borel function f : [2"]3 2W such that for any Eo tree
S C 2", we have that f"[S]3 = 2".
Proof: Define a Borel function f from [T]3 to 2', where T is any Eo-tree,
such that for any E0 subtree S C T we have that f([S]3) = 2". Given any set
{x1,x2, x3} ordered lexicographically, we may assume that all of the pairs (xi, j)
differ at infinitely many places for all 1 < i < j < 3. Let
m(xi, 2) = 3, m(x2,x3) = 1, m(x1, 3) = 2.
Since we are working on 2", there must be a least natural number ko and a
pair (x, x) selected from the set {xIx2, x3} such that x (ko) = x (ko); for n > 1 let
the natural number kn be the n-th smallest number such that for some pair (x!, xz),
we have that x'(kn) = x{(kn) and such that the pair (x' x) (x_-1 ,-1). Define
f(,(xi,, 23)(n) to be Im(xz, xZ) m(xZ, xn)I mod 2, where (xy, xn), (xn,xn) are such
that x](kn) = xz(kn) and x'(kn+1) = xn(kn+l)
Observe that if we divide 2W into m disjoint open balls, we get a function that
will hit every partition class for any E0 tree S C 2. Hence no partition theorem is
possible.
6.2 The Silver Forcing
In the following we study the Silver forcing (See [14] for more detail). The
Silver ideal is a variation of the Eo-ideal. Let G be the graph on 2W connecting two
binary sequences if and only if they differ in exactly one place. Let I be the ideal
o-generated by Borel G-independent sets.
Definition 6.2.1. A Silver tree is a nonempty tree T C 2W which is perfect, and
homogeneous in the sense that for every split node t E T every node compatible with
t is compatible with t^O or t^l, and {u : t^0u E T} = {u : tl"1u E T}.
Theorem 6.2.2. [14] Every analytic subset of 2W is either in the ideal I or it contains
all branches of some Silver tree.
The following results were generalized by Blass in a private conversation after
I told him how to construct a continuous function f : [2W]2 -+ 2 such that for any
Silver tree S we have that f"[S]2 = 2.
Theorem 6.2.3. There is a continuous function f : [T]n -+ m, where T is any Silver
tree, such that for any Silver tree S C T we have that f" [S] = m.
Proof: Define a continuous function f from [T]" to n, where T is a Silver
tree, the following way:
Given a Silver tree T and {xi : i < n} set of elements of T, let s be the lowest
branching point for the set {xi : i < n}, and let k be the number of l's in s modulo
m. Define f({(x : i < n}) = k.
It is easy to check that no homogeneous Silver subtree S C T for f is possible.
Theorem 6.2.4. There is a continuous function f : [T]" -+ w, where T is any Silver
tree, such that for any Silver tree S C T we have that f" [S]'" = w.
Proof: Given any Silver tree T and n-set {zi : i < n}, let s be the lowest
branching point of the n-set as before, now let k be the largest number m such that
2" divides the number of l's in s.
It is easy to check that this definition works.
Theorem 6.2.5. There is a Borel function f : [T]2 2", where T is any Silver tree,
such that for any Silver subtree S C T we have that f"[S]2 = 2".
Proof: Given any xZ, x2 E T, without loss we may assume that x1, x2 differ
at infinitely many places. Define the function f by induction: At the n-th stage look
at the n-th point that x1, x2 differ, if it is the case that xl(n) = 0, x2(n) = 1 then let
f(x1, z2)(n) = 0, otherwise, f(x1,x2)(n) = 1. If x1,x2 differ by only finitely many,
say, n places, let f(xl, x2)(m) = 0, whenever m > n.
6.3 The Packing Measures
In the following we define the packing measure on the space 2" with the metric
d(x, y) = 1r, where n is the least number such that x(n) 5 y(n). For any positive
real number h < 1, any set A C 2W, we say that {(xi, ri) : xi E A} is a 6-packing on
A if the open balls containing xi with radius ri are disjoint and 2ri < 6 for any i. Let
Po,(A) =sup{() rh : {(xa, r,) : i < w} forms a 6-packing }.
let
Po^(A) = limasoPOh6(A),
and define the packing h-measure for A C 2W by
00
ph(A) = inf{- Poh(Ai) : A C UiAi}.
i=1
Let Ih be the a -ideal generated by the subsets of 2" with finite packing h-
measure.
Theorem 6.3.1. For any perfect tree T, there is a continuous function f : [T]3 -+ 2,
such that for any Ih-positive S C T, we have that f" [S]3 = 2.
Proof: Fix a sequence of partitions (fn : n < w) such that each fn : [n]2 -+ 2
and each f, witness the smallest possible size of homogeneous set. By a combinatorial
theorem [2], which stated that
-k 2k 2\ k'
where nk is the least natural number such that for any partition f : [nk]2 -+ 2
there exists a homogeneous subset of size k, let dn be the smallest number such that
v/d2" > n. Given any {zo, x2} E [T]3, we will get exactly 2 forks, where a fork is an
s E 2<' such that both s'0 and s^l are initial segments of some xi, where i = 0, 1, 2.
Go to the level of the tree where the highest of the 2 forks are located, say, level n.
We have exactly 2 nodes, say s, t E 2n that are initial segments of xo, x1 or x2. Now
look at all the nodes at level n and let us say that we have totally m elements on
the n-th level of the tree ordered lexicographically and say that s is the i-th element
and t is the j-th element of the n-th level of the tree. Let f(xo, X1, x2) = fm(i, j).
Now it is easy to see that under this definition f is really continuous. We can easily
compute the weight at the n-th level, which is
S(( )n+1)h =m (( 1)n+l)h = a
The biggest size of homogeneous set that we are able to obtain is less than dn. Now
the size of the homogeneous set in the next level is at most d +2 by the last theorem.
So the weight on the next level is < (dn + 2) a (I)h. So the sum of all the weights
at all the levels is easily seen to converge to a finite value using the ratio test. Hence
it is impossible to have a Ih-positive subset that is homogeneous.
REFERENCES
[1] A. BLASS, A partition theorem for perfect sets, Proc. Amer. Soc., 82 (1981),
pp. 271-277.
[2] M. BONA, A walk through combinatorics, World Scientific, Hackensack, 2002.
[3] P. ERD6S AND R. RADO, A combinatorial theorem, J. London. Math. Soc.,
25 (1950), pp. 249-255.
[4] S. GESCHKE, M. KOJMAN, W. KUBIS, AND R. SCHIPPERUS, Convex decom-
positions in the plane and continuous pair coloring of the irrationals, Israel J.
Math., 131 (2002), pp. 285-317.
[5] F. GALVIN AND S. SHELAH, Some counterexamples in the partition calculus,
J. Combinatorial Theory Ser., A15 (1973), pp. 167-174.
[6] J. D. HALPERN AND H. LAUCHLI, A partition Theorem, Trans. Amer. Math.
Soc., 124 (1966), pp. 360-367.
[7] H. LEFMANN, Canonical partition behaviour of cantor spaces, in Irregularities
of Partitions, edited by G. Halisz and V.T. S6s, Springer-Verlag, New York,
1989, 93-105.
[8] R. LAVER, Products of infinitely many perfect trees, J. London Math. Soc., 29
(1984), pp. 385-396.
[9] J. MYCIELSKI, Independent sets in topological algebras, Fund. Math., 55 (1989),
pp. 139-147.
[10] J. MYCIELSKI, Algebraic independence and measure, Fund. Math., 61 (1967),
pp. 165-169.
[11] F. P. RAMSEY, On a problem of formal logic, Proc. London Math. Soc., 30
(1930), pp. 264-286.
[12] S. SHELAH, AND J. ZAPLETAL, Duality and the pcf theory, Mathematical
Research Letters, 9 (2002), pp. 585-595.
[13] S. TODORCHEVICH AND I. FARAH, Some application of the method of forcing,
Yenisei, Moscow, 1995.
[14] J. ZAPLETAL Descriptive set theory and definable forcing, AMS Memoirs, Prov-
idence, 2004.
BIOGRAPHICAL SKETCH
Yuan-Chyuan was born in Taiwan in 1973. He spent the first 25 years of his
life on that island. He came to Gainesville to study mathematics in 1998. In 2001
he began studying under Professor Jindrich Zapletal and working toward a doctoral
degree.
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of actor of Philophy.
Jn rich Zapletal Phairman
instant Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
Jean A. Larson
Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of ilosop
WifAm Mitc nlI
Professor of Mathematics
I certify that I have read this study and that in my opinion it conforms to
acceptable 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
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosop.
Greg Ray
Associate Professor of Philosophy
This dissertation was submitted to the Graduate Faculty of the Department of
Mathematics in the College of Liberal Arts and Sciences and to the Graduate School
and was accepted as partial fulfillment of the requirements for the degree of Doctor
of Philosophy.
May 2005
Dean, Graduate School
UNIVERSITY OF FLORIDA
1lU1111 III l IBII III I III II 111112 I
3 1262 08554 2412
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