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Functions Equivalent to Borel Measurable Ones

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Functions Equivalent to Borel Measurable Ones FUNCTIONS EQUIVALENT TO BOREL MEASURABLE ONES ANDRZEJ KOMISARSKI, HENRYK MICHALEWSKI, AND PAWELMILEWSKI Abstract. Let X and Y be two Polish spaces. Functions f and g : X ! Y are called equivalent if there exists a bijection ' from X onto itself such that g ◦ ' = f. Using a Theorem of J. Saint Raymond ([9]) we characterize functions equivalent to Borel measurable ones. This characterization answers a question asked by M. Morayne and C. Ryll-Nardzewski in [8]. 1. Introduction Let X and Y be two topological spaces. We say that functions f; g : X ! Y are equivalent if there is a bijection ' from X onto itself such that g ◦ ' = f. It is straightforward that f and g are equivalent if and only if for every y 2 Y there is jf −1(fyg)j = jg−1(fyg)j. Let X be a Polish (i.e. separable, completely metrizable) topological space and let B ⊂ P(X) be the family of Borel subsets of X. For a σ-ideal I ⊂ P(X) we say that a function g : X ! Y is σ(B; I)-measurable if g−1(U) is an element of the smallest σ-algebra containing B and I for any open set U ⊂ Y . In [8] Morayne and Ryll-Nardzewski proved that for a wide class of σ-ideals I ⊂ P(X), including the ideal of meager sets and the ideal of Lebesgue measure null-subsets of the interval [0; 1], a given function f : X ! R is equivalent to some σ(B; I)- measurable function g : X ! R if and only if fy 2 R : f −1(fyg) 6= ;g contains a topological copy of Cantor space or if there exists y 2 R such that jf −1(fyg)j = 2@0 . They also asked if there is a similar characterization of functions equivalent to Borel measurable ones. We answer this question using a recent result of J. Saint Raymond ([9]). 2. Notation All considered spaces are separable and metrizable. The Baire space, N is defined as the infinite countable product of N, equipped with the Tychonoff topology. Let X be a Polish space. A set A ⊂ X is analytic if it is a continuous image of N . A set C ⊂ X is coanalytic if its complement X n C is analytic. y Given a subset A ⊂ X × Y we define Ax = fy 2 Y :(x; y) 2 Ag, A = fx 2 X :(x; y) 2 Ag. Let N<N be the set of all finite sequences of the natural numbers. A nonempty subset T ⊂ N<N <N is a tree if 8n 8k ≤ n [(t0; : : : ; tn−1) 2 T ) (t0; : : : ; tk−1) 2 T ]. The space T r ⊂ 2N is the < set of all the trees endowed with the topology inherited from 2N N . T r is homeomorphic to the Cantor set. Let T be a tree. We say that (x0; x1;::: ) is a branch of T if for every n 2 N there is (x0; x1; : : : ; xn) 2 T . WF ⊂ T r is the the set of all well-founded (i.e. having no branches) trees and UB ⊂ T r is the set of the all trees with exactly one branch. 2000 Mathematics Subject Classification. Primary 54H05, 03E15, 28A05. Key words and phrases. coanalytic sets, equivalent functions, bimeasurable functions. 1 2 ANDRZEJ KOMISARSKI, HENRYK MICHALEWSKI, AND PAWELMILEWSKI We will need the following Lemma which is a known generalization of the Lusin Unicity Theorem ([1, 18.11]). We include a proof since we were unable to find a direct reference to the Lemma. Our method of proof is similar to a method used by Z. Koslova in [3]. Lemma 1. For any Borel subset B ⊂ X × Y of the product of Polish spaces X; Y , the sets B(n) = fx 2 X : jBxj = ng are coanalytic, n = 0; 1;:::; @0. Proof. Obviously B(0) is coanalytic. The Lusin theorem gives that B(1) is coanalytic. Let 2 ≤ n < @0 and let U1;U2;::: be a basis of Y . Put Vi = X × Ui; i = 1; 2 ::: Since x 2 B(n) , 9k1; : : : ; kn [(8i 6= j Vki \ Vkj = ;) n [ ^(8i ≤ n j(B \ Vki )xj = 1) ^ ((B n Vki )x = ;)]; i=1 the set B(n) is coanalytic. Consider now the case n = @0. Let f : F ! B be a continuous bijection defined on a closed set F ⊂ N (cf. [1, 13.7]). Put G = f(x; y; z): f(z) = (x; y)g ⊂ X ×(Y ×N ). Since jBxj = jGxj for all x 2 X, it is sufficient to show that G(@0) = fx 2 X : jGxj = @0g is coanalytic. Let us notice that G is closed and in particular all vertical sections of G are closed. Hence we may assume that Bx is closed for every x 2 X (we may replace B with G). The section Bx has infinitely many isolated points iff 8m 2 N 9k1; : : : ; km [(8i 6= j Vki \ Vkj = ;) ^ (8i = 1; : : : ; m j(B \ Vki )xj = 1)]: Hence lemma follows from the observation: x 2 B(@0) , Bx is countable and it has infinitely many isolated points, (the set fx 2 X : jBxj ≤ @0g is coanalytic, cf. [1, Theorem 29.19]). 3. Complete pairs of disjoint coanalytic sets In this section we deal with pairs of disjoint coanalytic sets. For short we call them just pairs. Following Louveau and Saint Raymond (see [6]) we introduce a notion of a reduction of pairs and a notion of a complete pair (a complete pair is called in [6] une pair reductrice pour Γ). Let X and Y be Polish spaces and let (A; B) and (C; D) be pairs in X and Y respectively. We say that a function r : X ! Y reduces (A; B) to (C; D) if for any x 2 X x 2 A if and only if f(x) 2 C and x 2 B if and only if f(x) 2 D: We say that r is a reduction of (A; B) to (C; D). A pair (C; D) in a Polish space Y is complete (see [6], a definition before Theorem 9 and Definition 1 in [9]) if for any pair (A; B) in the Baire space there exists a continuous reduction of (A; B) to (C; D). We will need the following result of Saint Raymond Theorem 2. ([9, Theorem 23]) The pair (W F; UB) is complete. FUNCTIONS EQUIVALENT TO BOREL MEASURABLE ONES 3 Let us notice, that due to the fact that any Polish space X can be embedded through a Borel measurable and bijective function into the Baire space, if (C; D) is a complete pair and (A; B) is a pair in X, then there exists a Borel measurable function reducing (A; B) to (C; D) (it is the superposition of the Borel measurable embedding and the reduction). Hence we obtain Corollary 3. If A,B are coanalytic and disjoint subsets of a Polish space Y . Then there exists a Borel function f : Y ! T r such that x 2 A if and only if f(x) 2 WF and x 2 B if and only if f(x) 2 UB. Using a method of [2] one can prove that if every pair (A; B) in any Polish space can be reduced to (C; D) by a Borel measurable function, then the pair (C; D) is complete. 4. Construction For a given countable family of disjoint, coanalytic subsets of a Polish space Y enumerated by the numbers f0;:::; @0g, we will construct (Corollary 7) a Borel function on a Polish space X such that each of the coanalytic sets is equal to the sets of the points in the space Y with preimages of the cardinality n 2 f0;:::; @0g. Furthermore, we will prove that the function may be chosen to be of the Baire first class. Lemma 4. Let Y be a Polish space and let B(0), B(1),..., B(@0) be subsets of Y . The following conditions are equivalent: (i) For every uncountable Polish space X there exists a Borel measurable function f : X ! −1 Y such that B(n) = fy 2 Y : jf (y)j = ng for every n 2 f0; 1;:::; @0g. (ii) For every uncountable Polish space X there exists an uncountable Borel set B ⊂ X ×Y y such that B(n) = fy 2 Y : jB j = ng for every n 2 f0; 1;:::; @0g. (iii) There exists an uncountable closed set F ⊂ N ×Y such that B(n) = fy 2 Y : jF yj = ng for every n 2 f0; 1;:::; @0g. (iv) There exists a Polish space X and an uncountable Borel set B ⊂ X × Y such that y B(n) = fy 2 Y : jB j = ng for every n 2 f0; 1;:::; @0g. Proof. Since the graph of a Borel measurable function f : X ! Y is a Borel subset of X × Y , obviously (i) implies (ii). Now, let X be an uncountable Polish space and let B ⊂ X × Y be an uncountable Borel set. To show that (ii) implies (iii), fix a closed subset of N and a continuous bijection g : A ! B (cf. [1, 13.7]) and define F = f(z; y) 2 N × Y : z 2 A; y = πY ◦ g(z); g(z) 2 Bg, where πY : X × Y ! Y is the projection. Clearly (iii) implies (iv). To prove the last implication ((iv))(i)) assume that X and B ⊂ X × Y satisfy (iv) and X0 is an arbitrary uncountable Polish space. Fix a Borel isomorphism g : X0 ! B (every two uncountable Borel subsets of Polish spaces are Borel isomorphic, cf [1, 15.6]).
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