Polishness of some topologies related to word or tree automata Olivier Finkel, Olivier Carton, Dominique Lecomte
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Olivier Finkel, Olivier Carton, Dominique Lecomte. Polishness of some topologies related to word or tree automata . 2018. hal-01593160v2
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Olivier Carton1, Olivier Finkel2 and Dominique LECOMTE3,4
July 15, 2018
1 Universite´ Paris Diderot, Institut de Recherche en Informatique Fondamentale, UMR 8243, Case 7014, 75 205 Paris Cedex 13, France [email protected]
2 Equipe de Logique Mathematique,´ Institut de Mathematiques´ de Jussieu-Paris Rive Gauche, CNRS et Universite´ Paris 7, France [email protected]
3 Projet Analyse Fonctionnelle, Institut de Mathematiques´ de Jussieu-Paris Rive Gauche, Univer- site´ Paris 6, France [email protected]
4 Universite´ de Picardie, I.U.T. de l’Oise, site de Creil, France
1998 ACM Subject Classification F.1.1 Models of Computation; F.1.3 Complexity Measures and Classes; F.4.1 Mathematical Logic; F.4.3 Formal Languages
Keywords and phrases Automata and formal languages; logic in computer science; infinite words; Buchi¨ automaton; regular ω-language; Cantor space; finer topologies; Buchi¨ topology; auto- matic topology; Polish topology; space of infinite labelled binary trees; Buchi¨ tree automaton; Muller tree automaton.
Abstract We prove that the Buchi¨ topology and the automatic topology are Polish. We also show that this cannot be fully extended to the case of the space of infinite labelled binary trees; in particular the Buchi¨ and the Muller topologies are not Polish in this case.
1 Introduction
This paper is a contribution to the study of the interactions between descriptive set theory and theoretical computer science. These interactions have already been the subject of many studies, see for instance [21, 45, 24, 42, 36, 33, 34, 39, 8, 38, 9, 13, 31, 11, 7, 41, 4, 28].
1 In particular, the theory of automata reading infinite words, which is closely related to infinite games, is now a rich theory which is used for the specification and the verification of non-terminating systems, see [15, 34]. The space ΣN of infinite words over a finite alphabet Σ, equipped with the usual Cantor topology τC , is a natural place to study the topological complexity of the ω-languages accepted by various kinds of automata. In particular, it is interesting to locate them with respect to the Borel and the projective hierarchies. However, as noticed in [35] by Schwarz and Staiger and in [18] by Hoffmann and Staiger, it turns out that for several purposes some other topologies on the space ΣN are useful, for instance for studying fragments of the first-order logic over infinite words, or for a topological characterization of the random infinite words (see also [17]). In particular, Schwarz and Staiger studied four topologies on the space ΣN of infinite words over a finite alphabet Σ, which are all related to automata, and refine the Cantor topology on ΣN: the Buchi¨ topology, the automatic topology, the alphabetic topology, and the strong alphabetic topology. Recall that a topological space is Polish if and only if it is separable, i.e., contains a countable dense subset, and its topology is induced by a complete metric. Classical descriptive set theory is about the topological complexity of the definable subsets of the Polish topological spaces, as well as the study of some hierarchies of topological complexity (see [19, 34] for the basic notions). The analytic sets, which are the projections of the Borel sets, are of particular importance. Similar hier- archies of complexity are studied in effective descriptive set theory, which is based on the theory of recursive functions (see [30] for the basic notions). The effective analytic subsets of the Cantor space (2N, τC ) are highly related to theoretical computer science, in the sense that they coincide with the sets recognized by some special kind of Turing machine (see [43]). We now give details about some topologies that we investigate in this paper. Let Σ be a finite alphabet with at least two symbols. We consider the following topologies on ΣN.
• the Buchi¨ topology τB, generated by the set BB of ω-regular languages,
• the automatic topology τA, generated by the set BA of τC -closed ω-regular languages (this 0 topology is remarkable because any τC -closed (or even τC -Π2) ω-regular language is accepted by some deterministic Buchi¨ automaton, [34]),
• the topology τδ, generated by the set Bδ of languages accepted by some unambiguous Buchi¨ Turing machine,
• the Gandy-Harrington topology τGH , generated by the set BGH of languages accepted by some Buchi¨ Turing machine.
In [35], Schwarz and Staiger prove that τB and τA are metrizable. The topology τB is separable, by definition, because there are only countably many regular ω-languages. It remains to see that it is completely metrizable to see that it is Polish. This is one of the main results proved in this paper.
Theorem 1 Let z ∈ {C, B, A, δ}. Then τz is Polish and zero-dimensional.
From this result, it is already possible to infer many properties of the space ΣN, equipped with the Buchi¨ topology (see [3] for an extended list). In particular, we get some results about the σ-algebra
2 generated by the ω-regular languages. It is stratified in a hierarchy of length ω1 (the first uncountable ordinal) and there are universal sets at each level of this hierarchy. Notice that this σ-algebra coincides with the σ-algebra of Borel sets for the Cantor topology. However, the levels of the Borel hierarchy 0 differ for the two topologies. For instance, an ω-regular set which is non-Π2 for the Cantor topology 0 is clopen (i.e., ∆1) for the Buchi¨ topology. Therefore the results about the existence of universal sets at each level of the σ-algebra generated by the ω-regular languages are really new and interesting. We also investigate, following a suggestion of H. Michalewski, whether it is possible to extend ω these results to the case of a space TΣ of infinite binary trees labelled with letters of the alphabet Σ. On the one hand, the automatic topology can be proved to be Polish in a similar way. On the other hand, we show that the Buchi¨ topology (generated by the set of regular tree languages accepted by some Buchi¨ tree automaton) and the Muller topology (generated by the set of regular tree languages accepted by some Muller tree automaton) are both non-Polish. However we prove that these two topologies have quite different properties: the first one is strong Choquet but not metrizable while the second one is metrizable but not strong Choquet.
2 Background
We first recall the notions required to understand fully the introduction and the sequel (see for example [34, 42, 19, 30]).
2.1 Theoretical computer science
A Buchi¨ automaton is a tuple A = (Σ, Q, Qi,Qf , δ), where Σ is the input alphabet, Q is the finite set of states, Qi and Qf are the sets of initial and final states, and δ is the transition relation. The transition relation δ is a subset of Q × Σ × Q. A run on some sequence σ ∈ ΣN is a sequence (qn)i∈ ∈ QN of states such that q0 is initial N (q0 ∈ Qi) and qi, σ(i), qi+1 is a transition in δ for each i ≥ 0. It is accepting if it visits infinitely often final states, i.e., qi ∈ Qf for infinitely many i’s. An input sequence σ is accepted if there exists an accepting run on α. The set of accepted inputs is denoted L(A). A set of infinite words is called ω-regular if it is equal to L(A) for some automaton A (see [34] for the basic notions about regular ω-languages, which are the ω-languages accepted by some Buchi¨ or Muller automaton). ABuchi¨ automaton is actually similar to a classical finite automaton. A finite word w of length n is accepted by some automaton A if there is sequence (qi)i≤n of n + 1 states such that q0 is initial (q0 ∈ Qi), qn is final (qn ∈ Qf ) and (qi, σ(i), qi+1) is a transition in δ for each 0 ≤ i < n. The set of accepted finite words is denoted by U(A). A set of finite words is called regular if it is equal to U(A) for some automaton A. Let U be a set of finite words, and V be a set of finite or infinite words. We recall that U · V = {u · v | u ∈ U ∧ v ∈ V }. An infinite word is ultimately periodic if it is of the form u · vω, where u, v are finite words. The ω-power of a set U of finite words is defined by
ω N N U = {σ ∈ Σ | ∃ (wi)i∈N ∈ U such that σ = w0 · w1 · w2 ···}. The ω-powers play a crucial role in the characterization of ω-regular languages (see [2]).
3 Theorem 2.1 (Buchi)¨ Let Σ be a finite alphabet, and L ⊆ ΣN. The following are equivalent:
1. L is ω-regular,
S ω 2. there are 2n regular languages (Ui)i In particular, each singleton {uvω} formed by an ultimately periodic ω-word is an ω-regular language. We now recall some important properties of the class of ω-regular languages (see [34] and [35]). Let Σ be a set, and Σ∗ be the set of finite sequences of elements of Σ. If w ∈ Σ∗, then w defines the usual basic clopen (i.e., closed and open) set Nw := {σ ∈ ΣN | w is a prefix of σ} of the ∗ Cantor topology τC (so BC := {∅} ∪ {Nw | w ∈ Σ } is a basis for τC ). Theorem 2.2 (Buchi)¨ The class of ω-regular languages contains the usual basic clopen sets and is closed under finite unions and intersections, taking complements, and projections (from a product alphabet onto one of its coordinates). We now turn to the study of Turing machines (see [6, 42]). A Buchi¨ Turing machine is a tuple M = (Σ, Γ, Q, q0,Qf , δ), where Σ and Γ are the input and tape alphabets satisfying Σ ⊆ Γ, Q is the finite set of states, q0 is the initial state, Qf is the set of final states, and δ is the transition relation. The relation δ is a subset of (Q × Γ) × (Q × Γ × {−1, 0, 1}). A configuration of M is a triple (q, γ, j) where q ∈ Q is the current state, γ ∈ ΓN is the content of the tape and the non-negative integer j ∈ N is the position of the head on the tape. Two configurations (q, γ, j) and (q0, γ0, j0) of M are consecutive if there exists a transition (q, a, q0, b, d) ∈ δ such that the following conditions are met. 1. γ(j) = a, γ0(j) = b and γ(i) = γ0(i) for each i 6= j. This means that the symbol a is replaced by the symbol b at the position j and that all the other symbols on the tape remain unchanged. 2. the two positions j and j0 satisfy the equality j0 = j + d. N A run of the machine M on some input σ ∈ Σ is a sequence (pi, γi, ji)i∈N of consecutive configurations such that p0 = q0, γ0 = σ and j0 = 0. The run is accepting if it visits infinitely often the final states, i.e., pi ∈ Qf for infinitely many i’s. The ω-language accepted by M is the set of inputs σ such that there exists an accepting run on σ. Notice that some other accepting conditions have been considered for the acceptance of infinite words by Turing machines, like the 1’ or Muller ones (the latter one was firstly called 3-acceptance), see [6, 42]. Moreover, several types of required behaviour on the input tape have been considered in the literature, see [43, 12, 10]. A Buchi¨ automaton A is in fact a Buchi¨ Turing machine whose head only moves forwards. This means that each of its transitions has the form (p, a, q, b, d) where d = 1. Note that the written symbol b is never read. 2.2 Descriptive set theory Classical descriptive set theory takes place in Polish topological spaces. We first recall that if d is a distance on a set X, and (xn)n∈N is a sequence of elements of X, then the sequence (xn)n∈N is 4 called a Cauchy sequence if 0 1 ∀k ∈ ∃N ∈ ∀p, p ≥ N d(x , x 0 ) < . N N p p 2k In a topological space X whose topology is induced by a distance d, the distance d and the metric space (X, d) are said to be complete if every Cauchy sequence in X is convergent. Definition 2.3 A topological space X is a Polish space if it is 1. separable (there is a countable dense sequence (xn)n∈N in X), 2. completely metrizable (there is a complete distance d on X which is compatible with the topol- ogy of X). The most classical hierarchy of topological complexity in descriptive set theory is the one given by the Borel classes. If Γ is a class of sets in metrizable spaces, then Γˇ := {¬S | S ∈ Γ} is the class of complements of elements of Γ. Recall that the Borel hierarchy is the inclusion from left to right in the following picture. 0 0 0 0 S 0 Σ1 =open Σ2 =(Π1)σ Σξ =( η<ξ Πη)σ 0 0 0 0 0 0 0 ∆1 =clopen ∆2 =Σ2 ∩ Π2 ··· ∆ξ =Σξ ∩ Πξ ··· 0 0 ˇ0 0 ˇ0 Π1 =closed Π2 =Σ2 Πξ =Σξ Above the Borel hierarchy sits the projective hierarchy, which the inclusion from left to right in the following picture. 1 1 1 1 1 Σ1 =analytic Σ2 =Projections of Π1 sets Σn+1 =Projections of Πn sets 1 ˇ1 1 ˇ 1 1 ˇ 1 Π1 =Σ1 Π2 =Σ2 Πn+1 =Σn+1 k Effective descriptive set theory is based on the notion of a recursive function. A function from N to l N is said to be recursive if it is total and computable. By extension, a relation is called recursive if its characteristic function is recursive. Definition 2.4 A recursive presentation of a Polish space X is a pair (xn)n∈N, d such that 1. (xn)n∈N is dense in X, 2. d is a compatible complete distance on X such that the following relations P and Q are recur- sive: m P (i, j, m, k) ⇐⇒ d(x , x ) ≤ , i j k + 1 m Q(i, j, m, k) ⇐⇒ d(x , x ) < . i j k + 1 5 A Polish space X is recursively presented if there is a recursive presentation of it. Note that the p 2 formula (p, q) 7→ 2 (2q + 1) − 1 defines a recursive bijection N → N. One can check that the coordinates of the inverse map are also recursive. They will be denoted n 7→ (n)0 and n 7→ (n)1 in the sequel. These maps will help us to define some of the basic effective classes. Definition 2.5 Let (xn)n∈N, d be a recursive presentation of a Polish space X. 1. We fix a countable basis of X: B(X, n) is the open ball B (x , ((n)1)0 ). d (n)0 ((n)1)1+1 0 2. A subset S of X is semirecursive, or effectively open (denoted S ∈ Σ 1) if [ S = B X, f(n), n∈N for some recursive function f. 0 3. A subset S of X is effectively closed (denoted S ∈ Π1 ) if its complement ¬S is semirecursive. 4. One can check that a product of two recursively presented Polish spaces has a recursive presen- tation, and that the Baire space NN has a recursive presentation. A subset S of X is effectively 1 0 N analytic (denoted S ∈ Σ1 ) if there is a Π1 subset C of X × N such that N S = π0[C] := {x ∈ X | ∃α ∈ N (x, α) ∈ C}. 1 5. A subset S of X is effectively co-analytic (denoted S ∈ Π1 ) if its complement ¬S is effectively 1 1 1 analytic, and effectively Borel if it is in Σ1 and Π1 (denoted S ∈ ∆1). 6. We will also use the following relativized classes: if X, Y are recursively presented Polish 1 1 spaces and y ∈ Y , then we say that A ⊆ X is in Σ1 (y) if there is S ∈ Σ1 (Y × X) such 1 that A = Sy := {x ∈ X | (y, x) ∈ S}. The class Π1 (y) is defined similarly. We also set 1 1 1 ∆1(y) := Σ1 (y) ∩ Π1 (y). The crucial link between the effective classes and the classical corresponding classes is as fol- lows: the class of analytic (resp., co-analytic, Borel) subsets of Y is equal to S Σ 1(α) (resp., α∈NN 1 S Π 1(α), S ∆1(α)). This allows to use effective descriptive set theory to prove results of α∈NN 1 α∈NN 1 classical type. In the sequel, when we consider an effective class in some ΣN with Σ finite, we will always use a fixed recursive presentation associated with the Cantor topology. The following result is proved in [43], see also [10]. Theorem 2.6 Let Σ be a finite alphabet, and L ⊆ ΣN. The following statements are equivalent: 1. L = L(M) for some Buchi¨ Turing machine M, 1 2. L ∈ Σ1 . We now recall the strong Choquet game played by two players on a topological space X. Players 1 and 2 play alternatively. At each turn i, Player 1 plays by choosing an open subset Ui and a point xi ∈ Ui such that Ui ⊆ Vi−1, where Vi−1 has been chosen by Player 2 at the previous turn. Player 2, plays by choosing an open subset Vi such that xi ∈ Vi and Vi ⊆ Ui. Player 2 wins the game if T V 6= ∅. We now recall some classical notions of topology. i∈N i 6 Definition 2.7 A topological space X is said to be • T1 if every singleton of X is closed, • regular if for every point of X and every open neighborhood U of x, there is an open neighbor- hood V of x such that the closure of V is contained in U, • second countable if its topology has a countable basis, • zero-dimensional if there is a basis made of clopen sets, • strong Choquet if X is not empty and Player 2 has a winning strategy in the strong Choquet game. Note that every zero-dimensional space is regular. The following result is Theorem 8.18 in [19]. Theorem 2.8 (Choquet) A nonempty, second countable topological space is Polish if and only if it is T1, regular, and strong Choquet. Let X be a nonempty recursively presented Polish space. The Gandy-Harrington topology on X 1 X is generated by the Σ1 subsets of X, and denoted τGH . By Theorem 2.6, this topology is also related to automata and Turing machines. As there are some effectively analytic sets whose complement is not analytic, the Gandy-Harrington topology is not metrizable (in fact not regular) in general (see 3E.9 in [30]). In particular, it is not Polish. Let Γ be a class of sets in Polish spaces. If Y is a Polish space, then we say that A ∈ Γ(Y ) is Γ-complete if, for each zero-dimensional Polish space X and each B ∈ Γ(X), there is f : X → Y continuous such that B = f −1(A). By Section 22.B in [19], if Γ is of the form Σ or Π in the Borel or the projective hierarchy, and if A is Γ-complete, then A is not in Γˇ. Theorem 22.10 in [19] gives a converse in the Borel hierarchy. 3 Proof of Theorem 1 The proof of Theorem 1 is organized as follows. We provide below four properties which en- sure that a given topological space is strong Choquet. Then we use Theorem 2.8 to prove that the considered spaces are indeed Polish. Let Σ be a countable alphabet. The set ΣN is equipped with the product topology of the discrete topology on Σ, unless another topology is specified. This topology is induced by a natural metric, called the prefix metric which is defined as follows. For σ 6= σ0 ∈ ΣN, the distance d is given by 1 d(σ, σ0) = , where r = min{n ∈ | σ(n) 6= σ0(n)}. 2r N When Σ is finite this topology is the classical Cantor topology. When Σ is countably infinite the topological space is homeomorphic to the Baire space NN. Let Σ and Γ be two alphabets. The function which maps each pair (σ, γ) ∈ ΣN × ΓN to the element σ(0), γ(0), σ(1), γ(1),... of (Σ × Γ)N is a homeomorphism between ΣN × ΓN and (Σ × Γ)N allowing us to identify these two spaces. 7 If Σ is a set, σ ∈ ΣN and l ∈ N, then σ|l is the prefix of σ of length l. We set 2 := {0, 1} and P∞ := {α ∈ 2N | ∀k ∈ N ∃i ≥ k α(i) = 1}. This latter set is simply the set of infinite words over the alphabet 2 having infinitely many 1’s. We will work in the spaces of the form ΣN, where Σ is a finite set with at least two elements. We consider a topology τΣ on ΣN, and a basis BΣ for τΣ. We consider the following properties of the family (τΣ, BΣ)Σ, using the previous identification of ΣN × ΓN and (Σ × Γ)N: (P1) BΣ contains the usual basic clopen sets Nw, (P2) BΣ is closed under finite unions and intersections, (P3) BΣ is closed under projections, in the sense that if Γ is a finite set with at least two elements and L ∈ BΣ×Γ, then π0[L] ∈ BΣ, (P4) for each L ∈ BΣ there is a closed subset C of ΣN × P∞ (i.e., C is the intersection of a τC -closed subset of the Cantor space ΣN × 2N with ΣN × P∞), which is in BΣ×2, and such that L = π0[C]. Theorem 3.1 Assume that a family (τΣ, BΣ)Σ satisfies Properties (P1)-(P4). Then the topologies τΣ are strong Choquet. Proof. We first describe a strategy τ for Player 2. Player 1 first plays σ0 ∈ ΣN and a τΣ-open neighborhood U0 of σ0. Let L0 in BΣ with σ0 ∈ L0 ⊆ U0. Property (P4) gives C0 with L0 = π0[C0]. 0 This gives α0 ∈ P∞ such that (σ0, α0) ∈ C0. We choose l0 ∈ N big enough to ensure that if 0 0 s0 := α0|l0, 0 then s has at least a coordinate equal to 1. We set w0 := σ0|1 and V0 := π0[C0 ∩ (Nw × N 0 )]. By 0 0 s0 Properties (P1)-(P3), V0 is in BΣ and thus τΣ-open. Moreover, σ0 ∈ V0 ⊆ L0 ⊆ U0, so that Player 2 respects the rules of the game if he plays V0. Now Player 1 plays σ1 ∈ V0 and a τΣ-open neighborhood U1 of σ1 contained in V0. Let L1 in BΣ with σ1 ∈ L1 ⊆ U1. Property (P4) gives C1 with L1 = π0[C1]. This gives α1 ∈ P∞ such that 1 1 1 1 (σ1, α1) ∈ C1. We choose l0 ∈ N big enough to ensure that if s0 := α1|l0, then s0 has at least one 0 0 coordinate equal to 1. As σ1 ∈ V0, there is α ∈ ∞ such that (σ1, α ) ∈ C0 ∩ (Nw × N 0 ). We 0 P 0 0 s0 0 0 0 0 0 0 choose l1 > l0 big enough to ensure that if s1 := α0|l1, then s1 has at least two coordinates equal to 1. We set w1 := σ1|2 and V1 := π0[C0 ∩ (Nw × N 0 )] ∩ π0[C1 ∩ (Nw × N 1 )]. Here again, V1 is 1 s1 0 s0 τΣ-open. Moreover, σ1 ∈ V1 ⊆ U1 and Player 2 can play V1. Next, Player 1 plays σ2 ∈ V1 and a τΣ-open neighborhood U2 of σ2 contained in V1. Let L2 in BΣ with σ2 ∈ L2 ⊆ U2. Property (P4) gives C2 with L2 = π0[C2]. This gives α2 ∈ P∞ such that 2 2 2 2 (σ2, α2) ∈ C2. We choose l0 ∈ N big enough to ensure that if s0 := α2|l0, then s0 has at least one 0 0 coordinate equal to 1. As σ2 ∈ V1, there is α ∈ ∞ such that (σ2, α ) ∈ C1 ∩ (Nw × N 1 ). We 1 P 1 0 s0 1 1 1 0 1 1 choose l1 > l0 big enough to ensure that if s1 := α1|l1, then s1 has at least two coordinates equal 00 00 0 0 to 1. As σ2 ∈ V1, there is α ∈ ∞ such that (σ2, α ) ∈ C0 ∩ (Nw × N 0 ). We choose l > l 0 P 0 1 s1 2 1 0 00 0 0 big enough to ensure that if s2 := α0|l2, then s2 has at least three coordinates equal to 1. We set 8 w2 := σ2|3 and V2 := π0[C0 ∩ (Nw × N 0 )] ∩ π0[C1 ∩ (Nw × N 1 )] ∩ π0[C2 ∩ (Nw × N 2 )]. Here 2 s2 1 s1 0 s0 again, V2 is τΣ-open. Moreover, σ2 ∈ V2 ⊆ U2 and Player 2 can play V2. l+1 n ∗ n n If we go on like this, we build wl ∈ Σ and sl ∈ 2 such that w0 ⊆ w1 ⊆ ... and s0 $ s1 $ ... N n N This allows us to define σ := liml→∞ wl ∈ Σ and, for each n ∈ N, αn := liml→∞ sl ∈ 2 . Note n n that αn ∈ P∞ since sl has at least l + 1 coordinates equal to 1. As (σ, αn) is the limit of (wl, sl ) as l goes to infinity and N × N n meets C (which is closed in ΣN × in the sense of Property (P4)), wl sl n P∞ (σ, αn) ∈ Cn. Thus \ \ \ \ σ ∈ π0[Cn] = Ln ⊆ Un ⊆ Vn, n∈N n∈N n∈N n∈N so that τ is winning for Player 2. 3.1 The Gandy-Harrington topology We have already mentioned the fact that the Gandy-Harrington topology is not Polish in general. However, it is almost Polish since it fulfills Properties (P1)-(P4). Let Σ be a finite alphabet with at least two elements and X be the space ΣN equipped with the X 1 topology τΣ := τGH generated by the family BΣ of Σ1 subsets of X. Note that the assumption of 0 Theorem 3.1 are satisfied. Indeed, (P1)-(P3) come from 3E.2 in [30]. For (P4), let F be a Π1 subset of X × NN such that L = π0[F ]. Let ϕ be the function from NN to 2N defined by ϕ(β) = 0β(0)10β(1)1 ... Note that ϕ is a homeomorphism from NN onto P∞, and recursive (which means that the relation ϕ(β) ∈ N(2N, n) is semirecursive in β and n). This implies that C := (Id ×ϕ)[F ] is suitable (see 3E.2 in [30]). 1 Note that τΣ is second countable since there are only countably many Σ1 subsets of X (see 3F.6 in [30]), T1 since it is finer than the usual topology by the property (P1), and strong Choquet by Theorem 3.1. One can show that there is a dense basic open subset ΩX of (X, τΣ) such that S ∩ ΩX is a clopen 1 subset of (ΩX , τΣ) for each Σ1 subset S of X (see [22]). In particular, (ΩX , τΣ) is zero-dimensional, and regular. As it is, just like (X, τΣ), second countable, T1 and strong Choquet, (ΩX , τΣ) is a Polish space, by Theorem 2.8. 3.2 The Buchi¨ topology Let Σ be a finite alphabet with at least two symbols, and X be the space ΣN equipped with the Buchi¨ topology τB generated by the family BB of ω-regular languages in X. Theorem 29 in [35] shows that τB is metrizable. We now give a distance which is compatible with τB. This metric was used in [17] (Theorem 2 and Lemma 21 and several corollaries following Lemma 21). A similar argument for subword metrics is in Section 4 in [18]. If A is a Buchi¨ automaton, then we denote |A| the number of states of A. We say that a Buchi¨ automaton separates x and y if and only if