# Descriptive Set Theory

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1 Martin Goldstern Institute of Discrete Mathematics and Geometry Vienna University of Technology Ljubljana, August 2011

2 Polish spaces Polish space = complete metric separable. Examples N = ω = {0, 1, 2,...}. R = real numbers. (E.g., Dedekind cuts) 2 ω = Cantor space = all infinite 01-sequences ω ω = Baire space = all infinite sequences of natural numbers. R n, R ω, (2 ω ) ω 2 ω.... many others

3 Polish spaces: convergence Polish space X = complete metric separable. To understand these spaces, we need to understand convergence in these spaces. a 1 = (5, 5, 5, 5, 5, 5...) a 2 = (3, 8, 0, 0, 0, 0,...) a 3 = (3, 6, 0, 5, 0, 0,...) a 3 = (3, 1, 4, 6, 0, 0,...) a 9 = (3, 1, 4, 1, 11, 1,...)... a ω = (3, 1, 4, 1, 5, 9, 2, 6,...). C X is closed, if convergent sequences in C have limit in C. U X is open iff X \ U is closed.

4 Continuum Hypothesis Cantor s Continuum Hypothesis (CH): Every infinite subset of the reals is either countable or equinumerous with R (or with 2 ω, i.e., contains a 1-1 copy of 2 ω ). (Hilbert s problem list, 1900: first problem.) The continuum hypothesis is neither provable nor refutable (from the usual axioms of set theory). (Cantor thought he had a proof.)

5 Perfect sets X=Polish, e.g. R or 2 ω. A nonempty set P X is perfect if P is closed P has no isolated points. (Every point in P is a limit of a sequence of distinct points in P.) Examples: [0, 1] R C [0, 1], the Cantor set. 2 ω C. Fact S X contains a perfect set iff S contains a Cantor set, i.e., iff there is a continuous injective map from 2 ω into S. Hence: If X contains a perfect set, then X is uncountable.

6 Fact S X contains a perfect set iff S contains a Cantor set, i.e., iff there is a continuous injective map from 2 ω into S. Corollary If X contains a nonempty perfect set, then X is uncountable. Question Is the converse true? Effective continuum hypothesis: Every infinite subset of the reals is either countable or contains a perfect set, i.e., contains a 1-1 continuous copy of 2 ω.

7 Effective Continuum Hypothesis Effective continuum hypothesis: Every infinite subset of the reals is either countable or contains a perfect set, i.e., contains a 1-1 continuous copy of 2 ω. The effective continuum hypothesis is (strictly speaking) false, but in practice often true. Refutable in ZFC. But the counterexample is complicated. Not refutable in ZF without the axiom of choice. (disclaimer... ) Provable for many definable sets in ZFC, even in ZF. Provable for most definable sets in ZFC+large cardinals. Thesis: Never claim that a set is merely uncountable when you can in fact show that it contains a perfect set.

8 Perfect set property A class C of subsets of X has the perfect set property if each uncountable set in C contains a perfect set. (I.e., if all sets in C satisfy the effective CH.) EXAMPLES: open sets closed sets Borel sets analytic sets Note: being countable is a positive property. Being uncountable is a negative property. But contains a perfect set is positive!

9 Borel sets. Π 0 ω Σ 0 ω 0 ω... G δ = Π 0 2 Σ 0 2 = F σ closed = Π Σ 0 1 = open 0 1

10 Borel sets, examples We consider subsets of 2 ω. {x 2 ω : x(2) = 0 & x(3) = 1} is clopen, i.e., 0 1. The set of all eventually zero sequences is countable, hence F σ, i.e., Σ 0 2. The set U of all uniformly distributed sequences is Π 0 3 : n x U k m n m : i=0 x(i) 1 n 2 1 k U = 1 0 = Π 0 1 = Σ 0 2 = Π0 3 k ω m ω m n k ω m ω k ω

11 The perfect set property for closed sets Theorem (Cantor-Bendixson) Let X be a closed uncountable set. (Subset of R n, of 2 ω, etc.) Then there is a perfect set P such that C := X \ P is countable. Proof sketch. Let C 0 be the set of isolated points of X. C 0 must be countable, and X (1) := X \ C 0 is still closed uncountable. Continue for infinitely many steps: X X (1) X (2). Stop when C n = (then X (n) is perfect). If you are not done after ω steps, let X (ω) := X (n), and continue. This process (of taking Cantor-Bendixson derivatives ) must stop at a countable ordinal.

12 Analytic sets An analogy: Borel sets have a flavour that is similar to recursive (=decidable) subsets of the natural numbers. (E.g.: closed under complements.) Analytic sets correspond to r.e. sets (recursively enumerable, computably enumerable, semi-decidable). (Not closed under complements.) Another analogy: Borel sets have a flavour that is similar to sets in P = polynomially decidable sets. Analytic sets correspond to sets in NP.

13 Analytic sets, several definitions Let X be a Polish space (R n, ω ω, etc). Let S X be nonempty. Then the following are equivalent: 1. There is a continuous F : ω ω X with F [ω ω ] = S. 2. There is a continuous F : ω ω X and a closed set C ω ω with F [C] = S. 3. As above, but C may be a Borel set. 4. There is a closed (or Borel) subset of ω ω X whose projection to X is exactly S.

14 Analytic sets, THE definition Let X be a Polish space. S X is analytic if S = or there is a continuous map F : ω ω X with F [ω ω ] = S. (We should check X itself is analytic under this definition.) Rule of thumb: Anything involving only k ω, k ω, k ω, k ω is a Borel set. If you have to use y ω ω or r R, then analytic and usually not Borel. Example {x ω ω (n 1 < n 2 < ) : x n1 x n2 } is analytic, not Borel. {x ω ω (n 1 < n 2 < ) : x n1 = x n2 = } is Borel.

15 Closure properties The family of analytic sets contains all closed sets and is closed under countable unions (hence contains all closed sets), countable intersections (hence contains all Borel sets), continuous images (by definition), continuous preimages. But NOT under complement. Empirical fact: (100-ε)% of all sets considered in analysis are Borel sets. (100-ε)% of the remaining ε% are analytic (or co-analytic).

16 The perfect set property for analytic sets Let F : ω ω X be continuous, and assume that A := F [ω ω ] is uncountable. We claim that A contains a perfect set. More precisely: we will find a copy of 2 ω in ω ω on which F is 1-1. For s ω <ω, we write [s] for the {x : x ω ω, s x} (= all branches extending the node s.) For s ω <ω, we write F [s] for the set {f (x) : x ω ω, s x}. We will show our claim under the additional assumption that F [s] has more than one element, for each s. (This can easily be arranged: Iteratively remove those s for which this is not true; with each s we remove from ω ω all elements extending s, but all of them have the same image. Since only countably many s were removed, only countably many elements from A were removed.)

17 The perfect set property for analytic sets, part 2 Assumption: F : ω ω X is continuous, and for each s ω <ω the set F [s] := {f (x) : x ω ω, s f } has > 1 elements. (NOTE: for different s, the sets F [s] are not necessarily disjoint. If they were, everything would be trivial.) Construction: 1. Let t :=, the empty sequence. 2. We find two disjoint nonempty open sets U 0, U 1 in F[t]. 3. We find t 0, t 1 ω <ω, extending t, such that F [t l ] U l. (Use continuity of F.) 4. Continue by splitting t 0 to t 00 and t 01, and t 1 to t 10, t 11, using disjoint open sets U 00, U 01, U 10, U etc.

18 The perfect set property for analytic sets, part 3 What we have: For each finite 0, 1-sequence s we have found a finite sequence t s ω <ω such that: If s extends s, then t s extends t s If s and s are incompatible, then also t s and t s are incompatible, and moreover: F [t s ] and F[t s are disjoint. What we want: A continuous 1-1 map from 2 ω into F [ω ω ]. Easy: For each x 2 ω, say x = (a, b, c,...), let t x be the limit of t, t a, t ab, t abc. The map x F (t x ) is 1-1 and continuous.

19 Lebesgue measure Every open subset U of R is a union of (finitely or countably many) disjoint intervals. The measure µ(u) of U is the sum of the lengths of these intervals. The measure µ is defined on all Borel sets, and it is σ-additive. A set N R has measure zero if for all ε there exists an open set U of measure < ε with N U. A set S R is measurable iff there exists a Borel (or Σ 0 2 ) set B such that B S has measure 0. We set µ(s) := µ(b).

20 Measure, continued Theorem All analytic sets are measurable. Definition Let X, Y be measure spaces. A function g : X Y is measurable if g 1 (U) is measurable for all open sets U Y. Measurable functions from some unspecified space X into our space Y are called random variables in statistics.

21 Uniformisation Theorem (AC) Let S X Y, and assume πs = X, i.e., x X : S x := {y Y (x, y) S}. Then there is a function g : X Y whose graph is contained in S, i.e., g(x) S x for all x. (This theorem is equivalent to the axiom of choice.) Theorem ( Uniformisation theorem ) Assume that S from above is nice. Then we can find a nice g. (Usually without the axiom of choice.)

22 Jankov-von Neumann Theorem Let S X Y be analytic, πs = X. Then there is a measurable function g S. For the proof, we may assume that S is closed: Let S = F [ω ω ], and Ŝ := {(x, y, z) X Y ω ω (x, y) = F (z)} X (Y ω ω ) If we can find a function ĝ : X Y ω ω uniformizing Ŝ, then we get g uniformizing S by projection. (Details omitted.)

23 Closed subsets of ω ω Let C ω ω. The set T C := {c n : c C, n ω} ω <ω is a tree. (Downward closed set of finite sequences.) (Moreover, T C has no leaves = nodes without extensions.) We write [T C ] for the set of its branches: [T C ] := {x ω ω : n x n T C } C. It is easy to check that [T C ] = closure of C. Hence: Closed sets = trees without leaves.

24 Jankov-von Neumann for closed sets Theorem Let C ω ω ω ω be closed, πc = X. Then there is a nice (in fact: measurable) function g C. Proof sketch. For each x ω ω, the set C x := {y : (x, y) C} is closed, so C x = [T x ] for some tree T x. Let g(x) := the leftmost branch of [T x ]. This is a nicely defined function, hence measurable. (Need to check details.)

25 An application Fact Let (B n : n ω) be a family of sets of measure zero. Then n B n has measure zero. (Trivially.) But: There are families (B x : x ω ω ) of measure zero sets such that in x B x does not have measure zero. (Easy to find.) Theorem Let (B x : x ω ω ) be a nice family of sets Y with the following property: Whenever x 1 x 2, then B x1 B x2. Then the set x ω ω B x still has measure zero. ( nice means that the set {(x, y) : y B x } is a Borel (or analytic) set. By x 1 x 2 I means that x 1 (n) x 2 (n) for all n.)

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