# Lights and Darks of the Star-Free Star

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1 Lights and Darks of the Star-Free Star Edward Ochmański & Krystyna Stawikowska Nicolaus Copernicus University, Toruń, Poland

2 Introduction: star may destroy recognizability In (finitely generated) trace monoids we have: singletons are in REC REC is closed under union REC is closed under catenation REC is included in RAT Corollary: If a rational language is not recognizable, recognizability was broken by a star operation.

3 History: questions on recognizable star Rational expressions are often more convenient (e.g. for inductive proofs) than the automata-oriented or algebraic descriptions of recognizable languages Definition: Recognizable star is the classical star, working only if its result is recognizable (and undefined otherwise) Questions: Does recognizable star produce (jointly with union and catenation) the whole class REC?

4 History: recognizable stars in free and trace monoids Questions: Does recognizable star produce (jointly with union and catenation) the whole class REC? Answer 1 (Kleene): In finitely generated free monoid YES. Any star keeps recognizability! Answer 2: In arbitrary trace monoid YES. The connected star keeps recognizability and builds the whole REC.

5 Monoids, languages, free monoids Monoid (M, ) a set M with an associative operation (called product) and a neutral element ε (called unit). Language in (M, ) any subset of M. Atomic languages (atoms) empty language and singletons {m}, for m M. Free Monoid (A*, ) set of all finite words (including ε) over an alphabet A, with concatenation.

6 Operations on languages Set-theoretical operations: union X Y intersection X Y difference X \ Y complement X = M \ X Algebraic operations: product: X Y = { xy x X, y Y} power: X 0 = {ε}, X n+1 = XX n star: X* = U{X n n=0,1,...}

7 Rational and star-free languages Rational languages: languages built up from atoms with operations of union, product and star. Rational expressions: expressions describing such construction. RAT(M): the class of rational subsets of M. Star-free languages: languages built up from atoms with operations of union, product and complement. Star-free expressions: expressions describing such construction. SF(M): the class of star-free subsets of M.

8 Syntactic monoid, recognizable and aperiodic languages syntactic congruence L M M of a language L M: u L v iff ( x,y M) xuy L xvy L syntactic monoid of a language L M: A language L M is: quotient monoid M L =M/ L recognizable (REC) iff its syntactic monoid M L is finite; aperiodic (AP) iff its syntactic monoid M L is finite and. aperiodic, i.e. ( n)( x M L ) x n =x n+1.

9 Star-Free = Aperiodic (in A*) Theorem (Schützenberger 1965): In finitely generated free monoids SF(A*) = AP(A*) i.e. a language is star-free iff it is aperiodic Example: The language (aa)* has syntactic monoid: ε a ε ε a It is not aperiodic, a a ε as a n a n+1 for any n. By Schützenberger Theorem, (aa)* is not star-free.

10 Example: M={a,b}*, L=(ab)* The language (ab)* is star-free (ab)* = (b a aa bb ) Syntactic monoid of (ab)* ε a b ab ba 0 ε ε a b ab ba 0 a a 0 ab 0 a 0 b b ba 0 b 0 0 ab ab a 0 ab 0 0 ba ba 0 b 0 ba a 2 = a 3 = 0 b 2 = b 3 = 0 (ab) 2 = (ab) 3 = ab (ba) 2 = (ba) 3 = ba ε 2 = ε 3 = ε 0 2 = 0 3 = 0 n = 2 Hence (ab)* is aperiodic

11 Operation of STAR-FREE STAR Star-free star operation: L = L* if L* is star-free undefined otherwise SFS-expression: a rational expression, built from atoms with symbols of union, product and star-free star SFS-language: a language built up from atoms with operations of union, product and star-free star

12 Question: SFS = SF? Inclusion holds in any monoid, since union, product and star-free star preserve star-freeness. Inclusion is not general; there are monoids in which it does not hold. Example: (Z,+) integers with addition SF = { X Z X or X is finite } SFS = { X Z X=Z or X is finite }

13 Star-free star in free monoids Theorem (O/S 2005): In finitely generated free monoids SFS = SF Inclusion holds in any monoid. The proof of is based on McNaughton/Yamada (1960) construction of regular expressions for automata and uses the Schützenberger Theorem.

14 Many-sided characterization of Star-Free Trace Languages SF(A*)=SFS(A*)

15 Traces and trace languages Concurrent alphabet pair (A,I), where A is a finite alphabet, I A A is a symmetric and irreflexive independence relation; complement of I, the relation D=A A I, is called dependency. Trace monoid M=A*/ I quotient of A* by the least congruence containing the relation {ab ba aib}. Traces members of trace monoids. Trace languages subset of trace monoids. Free monoid trace monoid with I=.

16 Lemma on closing product Flattening of a trace language: UT = { w A* [w] T } Closure of a word language: L = U[L] = { w A* [w] [L] } L is closed iff L=L Lemat: If K,L A* are closed and star-free, then KL is star-free.

17 Flat characterization of star-free trace languages SF(A*)=SFS(A*) Lemma on closing product Schützenberger Theorem T SF(A*/I) iff UT SF(A*)

18 Many-sided characterization of Star-Free Trace Languages SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*)

19 Aperiodic characterization of star-free trace languages Theorem (Guaiana/Restivo/Salemi 1992): In any trace monoid, T is star-free iff T is aperiodic. Proof: T AP(A*/ I ) T SF(A*/ I ) UT AP(A*) UT SF(A*)

20 Many-sided characterization of Star-Free Trace Languages SF(A*/I)=AP(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*)

21 Logic for traces (Thomas 1989) (A,I) concurrent alphabet, w=a 1 a n A*. Model for the trace [w] A*/ I is the trace-graph V,E,λ where V={x 1 x n } vertices, E V V edges and λ:v A is s.t. ( i) λ(x i )=a i and x i Ex j iff i<j & a i Da j First order formula is build up from atomic formulas x=y, xey and λ(x)=a with logical connectives and quantifiers.

22 First order definability T(Ψ)={α A*/ I Ψ is satisfied in α} trace language defined by the sentence Ψ. Trace language T is first-order definable iff there is a first-order sentence Ψ s.t. T=T(Ψ). FO(A*/ I ) the class of first-order definable languages in the trace monoid A*/ I. Theorem (McNaughton/Papert 1971): In finitely generated free monoids SF(A*)=FO(A*)

23 Lexicographic representation (A,<,I) ordered concurrent alphabet Lexicographic representative of trace α A*/ I a word Lex(α) A*, lexicographically first in α. Lex(T) = { Lex(α) A* α α T } lexicographic representation of the trace language T. LEX = Lex (A*/ I ) lexicographic representation of the trace monoid. LEX = A* U{A*b(I a )*aa* aib a<b}, where I a ={c A aic}, hence LEX is star-free.

24 Flat and Lex characterizations of first-order trace languages Theorem (Ebinger/Muscholl 1993): For any trace language T, the following statements are equivalent: (1) T is first-order definable in A*/ I (2) UT is first-order definable in A* (3) Lex(T) is first-order definable in A*

25 Many-sided characterization of Star-Free Trace Languages SF(A*/I)=AP(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*)

26 Logical characterization of star-free trace languages Theorem (Ebinger/Muschol 93, Diekert/Metivier 97): A trace language is star-free if and only if it is first-order definable Proof. T SF(A*/ I ) iff UT SF(A*) iff UT FO(A*) iff T FO(A*/ I )

27 Many-sided characterization of Star-Free Trace Languages SF(A*/I)=AP(A*/I) SF(A*/I)=FO(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*)

28 Lexicographic characterization of star-free trace languages Theorem: A trace language is star-free in A*/ I if and only if its lexicographic representation is star-free in A* Proof. T SF(A*/ I ) iff T FO(A*/ I ) iff Lex(T) FO(A*) iff Lex(T) SF(A*)

29 Many-sided characterization of Star-Free Trace Languages SF(A*/I)=AP(A*/I) SF(A*/I)=FO(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*) T SF(A*/I) iff Lex(T) SF(A*)

30 Star-free star in trace monoids Lemma 1: If L SFS(A*) and L LEX, then [L] SFS(A*/ I ) Proof. Structural induction on SFS-expressions. Lemma 2: If UT SFS(A*), then T SFS(A*/ I ) Theorem: In trace monoids SFS = SF Proof. T SF(A*/ I ) UT SF(A*) UT SFS(A*) T SFS(A*/ I ) T SF(A*/ I )

31 Many-sided characterization of Star-Free Trace Languages SF(A*/I)=AP(A*/I) SF(A*/I)=FO(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*) SF(A*/I)=SFS(A*/I) T SF(A*/I) iff Lex(T) SF(A*)

32 Problems 1-3 Problem 1: To find syntactically formulated conditions for star-free star in trace monoids. Problem 2: In which monoids SFS=SF? Does it hold in concurrency monoids of Droste?

33 Decision Problems Theorem (Muscholl/Petersen 1996): The problem "Is T star-free?" is decidable for rational trace languages only in trace monoids with transitive independency. Problem 3 (star problem for s-f trace languages): Is the question Is T* star-free? decidable for: star-free trace languages? finite trace languages?

34 A part without logic SF(A*/I)=AP(A*/I) SF(A*/I)=FO(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*) SF(A*/I)=SFS(A*/I) T SF(A*/I) iff Lex(T) SF(A*)

35 A part without logic SF(A*/I)=AP(A*/I) SF(A*/I)=FO(A*/I) SF(A*)=SFS(A*) T SF(A*/I) iff UT SF(A*) T FO(A*/I) ) iff UT FO(A*) iff Lex(T) FO(A*) SF(A*/I)=SFS(A*/I) T SF(A*/I) iff Lex(T) SF(A*)

36 Problem 4 avoiding logic Problem 4: To prove the non-logic part of To prove the non-logic part of the characterization without logic

37 LSF languages lexicographic product: X Y = XY if XY LEX else undefined lexicographic complement: X" = LEX X LSF = the class of word languages that are built from atoms with union, lexicographic product and lexicographic complement

38 Question: LSF = SF inside LEX? Lemma: If L LSF, then [L] SF(A*/ I ) Proof: By lemma on closing product. By definition: If L LSF, then L SF and L LEX Question: Does the converse hold?

39 Transitively oriented alphabets An ordered alphabet (A,<,I ) is transitively oriented iff < I is transitive. Lemma: If L SF and L LEX, then L LSF Proposition: LSF = {L A* L SF and L LEX} if (A,<,I ) is transitively oriented Problem 4a: Does the proposition hold for any ordered alphabet?

40 Example D: a c b LEX = (a*b*c)*a*b* = a*b*(ca*b*)* a* = LEX (LEX c LEX LEX b) b* = LEX (LEX c LEX a LEX) c* = LEX ((LEX LEX b) a LEX LEX b (LEX a LEX)) (a c)* = LEX (LEX b LEX bc LEX) a*b* = LEX LEX c LEX (ac)* = LEX (c LEX (LEX LEX b) a LEX cc LEX (LEX LEX b) aa LEX)

41 How is LSF in the Pentagon? Pentagon cannot be transitively oriented D: What about LSF in the pentagon? Does a* belong to LSF?

42 Publications Edward Ochmański, Krystyna Stawikowska: On Closures of Lexicographic Star-Free Languages, Proceedings of AFL 2005, pp , University of Szeged, Edward Ochmański, Krystyna Stawikowska: Star-Free Star and Trace Languages, Fundamenta Informaticae 72, pp , IOS Press Krystyna Stawikowska, Edward Ochmański: On Star-Free Trace Languages and their Lexicographic Representations, Proceedings of LATA 2007, pp , Universitat Rovira i Virgili, Tarragona, Spain, Edward Ochmański, Krystyna Stawikowska: A Star Operation for Star-Free Trace Languages, Proceedings of DLT 2007, LNCS 4588, pp , Springer Krystyna Stawikowska: Word Languages Inducing Recognizable and Star-Free Trace Languages, PhD Thesis (in Polish), Toruń-Warszawa, 2007.

43 the end

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