This paper contributes to the techniques of topo-algebraic recognition for languages beyond the regular setting as they relate to logic on words. In particular, we provide a general construction on recognisers corresponding to adding one layer of various kinds of quantifiers and prove a corresponding Reutenauer-type theorem. Our main tools are codensity monads and duality theory. Our construction hinges on a measure-theoretic characterisation of the profinite monad of the free S-semimodule monad for finite and commutative semirings S, which generalises our earlier insight that the Vietoris monad on Boolean spaces is the codensity monad of the finite powerset functor.

If $\mathcal {A}$ is a finite set (alphabet), the shift dynamical system consists of the space $\mathcal {A}^{\mathbb {N}}$ of sequences with entries in $\mathcal {A}$, along with the left shift operator S. Closed S-invariant subsets are called subshifts and arise naturally as encodings of other systems. In this paper, we study the number of ergodic measures for transitive subshifts under a condition (‘regular bispecial condition’) on the possible extensions of words in the associated language. Our main result shows that under this condition, the subshift can support at most $({K+1})/{2}$ ergodic measures, where K is the limiting value of $p(n+1)-p(n)$, and p is the complexity function of the language. As a consequence, we answer a question of Boshernitzan from 1984, providing a combinatorial proof for the bound on the number of ergodic measures for interval exchange transformations.

The concept of the algorithmic complexity of crystals is developed for a particular class of minerals and inorganic materials based on orthogonal networks, which are defined as networks derived from the primitive cubic net (pcu) by the removal of some vertices and/or edges. Orthogonal networks are an important class of networks that dominate topologies of inorganic oxysalts, framework silicates and aluminosilicate minerals, zeolites and coordination polymers. The growth of periodic orthogonal networks may be modelled using structural automata, which are finite automata with states corresponding to vertex configurations and transition symbols corresponding to the edges linking the respective vertices. The model proposed describes possible relations between theoretical crystallography and theoretical computer science through the theory of networks and the theory of deterministic finite automata.

The avoidability of binary patterns by binary cube-free words is investigated and the exact bound between unavoidable and avoidable patterns is found. All avoidable patterns are shown to be D0L-avoidable. For avoidable patterns, the growth rates of the avoiding languages are studied. All such languages, except for the overlap-free language, are proved to have exponential growth. The exact growth rates of languages avoiding minimal avoidable patterns are approximated through computer-assisted upper bounds. Finally, a new example of a pattern-avoiding language of polynomial growth is given.

We study the avoidance of Abelian powers of words and consider three reasonablegeneralizations of the notion of Abelian power to fractional powers. Our main goal is tofind an Abelian analogue of the repetition threshold, i.e., a numericalvalue separating k-avoidable and k-unavoidable Abelianpowers for each size k of the alphabet. We prove lower bounds for theAbelian repetition threshold for large alphabets and all definitions of Abelian fractionalpower. We develop a method estimating the exponential growth rate of Abelian-power-freelanguages. Using this method, we get non-trivial lower bounds for Abelian repetitionthreshold for small alphabets. We suggest that some of the obtained bounds are the exactvalues of Abelian repetition threshold. In addition, we provide upper bounds for thegrowth rates of some particular Abelian-power-free languages.

An ever present, common sense idea in language modelling research is that, for aword to be a valid phrase, it should comply with multiple constraints atonce. A new language definition model is studied, based on agreement or consensusbetween similar strings. Considering a regular set of strings over a bipartitealphabet made by pairs of unmarked/marked symbols, a match relation isintroduced, in order to specify when such strings agree. Then a regular setover the bipartite alphabet can be interpreted as specifying another languageover the unmarked alphabet, called the consensual language. A word is in theconsensual language if a set of corresponding matching strings is in theoriginal language. The family thus defined includes the regular languages andalso interesting non-semilinear ones. The word problem can be solved inNLOGSPACE, hence in P time. The emptiness problem is undecidable. Closure properties areproved for intersection with regular sets and inverse alphabetical homomorphism.Several conditions for a consensual definition to yield a regular language arepresented, and it is shown that the size of a consensual specification ofregular languages can be in a logarithmic ratio with respect to a DFA. Thefamily is incomparable with context-free and tree-adjoining grammar families.

This paper discusses context-free rewriting systems inwhich there exist two disjoint finite sets of rules, and a symbol, referred to as a condition of applicability, is attached to each rule in either of these two sets. In one set, a rule with a symbol attached to it is applicable if the attached symbol occurs in the current rewritten string while in the other set, such a rule is applicable if the attached symbol does not occur there. The present paper demonstrates that these rewriting systems are computationally complete. From this main result, the paper derives several consequences and solves several open problems.

A parallel communicating automata system consists of several automata working independently in parallel and communicating with each other by request with the aim of recognizing a word. Rather surprisingly, returning parallel communicating finite automata systems are equivalent to the non-returning variants. We show this result by proving the equivalence of both with multihead finite automata. Some open problems are finally formulated.

It is known that the class of factorizing codes, i.e.,codes satisfying the factorization conjectureformulated by Schützenberger, isclosed under two operations:the classicalcomposition of codes and substitutionof codes.A natural question which arisesis whethera finite set Oof operations existssuch that each factorizingcode can be obtained by usingthe operations inO and starting with prefix or suffix codes.O is named herea complete setof operations (for factorizing codes).We show that composition and substitutionare not enough in order to obtaina completeset. Indeed, we exhibit a factorizing code over a two-letter alphabet A = {a,b}, precisely a 3-code, which cannot beobtained by decomposition or substitution.

We give necessary and sufficient conditions for a language to be the language of finite words that occur infinitely many times in an infinite word.

Circular splicing has been very recently introducedto model a specific recombinant behaviourof circular DNA, continuing the investigation initiatedwith linear splicing. In this paper we restrict ourstudy to therelationship between regular circular languagesand languages generated by finite circular splicing systemsand provide some results towards a characterizationof the intersection between these two classes.We consider the class of languages X*, calledhere star languages, which are closed under conjugacyrelation and with X being a regular language.Usingautomata theory and combinatorial techniques on words, weshow that for a subclass of star languagesthe corresponding circular languagesare (Paun) circular splicing languages.For example, star languages belongto this subclass when X* is a free monoidor X is a finite set.We also prove thateach (Paun) circular splicing language Lover a one-letter alphabet has the formL = X+ ∪ Y, with X,Y finite sets satisfyingparticular hypotheses.Cyclic and weak cyclic languages,which will be introduced in this paper, show that thisresult does not hold when we increase the size ofalphabets, even if we restrict ourselves to regular languages.

The basic framework of domain μ-calculus was formulated in [39] more than ten years ago.This paper provides an improved formulation of a fragment of the μ-calculus without function space or powerdomain constructions,and studies some open problemsrelated to this μ-calculus such asdecidability and expressive power.A class of language equations is introducedfor encoding μ-formulas in order toderive results related to decidability and expressive power of non-trivial fragments of the domain μ-calculus.The existence and uniqueness of solutions tothis class of language equations constitute an important component of this approach.Our formulation is based on the recent work of Leiss [23], who established a sophisticated framework for solving language equationsusing Boolean automata(a.k.a. alternating automata [12,35]) and a generalized notion of language derivatives.Additionally, the early notion of even-linear grammars is adopted here totreat another fragment of the domain μ-calculus.

In this paper we introduce a sharpening of the Parikh mapping and investigate its basic properties.The new mapping is based on square matrices of a certain form. The classical Parikh vectorappears in such a matrix as the second diagonal.However, the matrix product gives more information abouta word than the Parikh vector. We characterize the matrixproducts and establish also an interesting interconnectionbetween mirror images of words and inverses of .

We investigate the complexity of languages described by some expressionscontaining shuffle operator and intersection. We show that deciding whetherthe shuffle of two words has a nonempty intersection with a regular set(or fulfills some regular pattern) is NL-complete.Furthermore we show that the class of languages of the form $L\cap R$ ,with a shuffle language L and a regular language R, containsnon-semilinear languages and does not form a family of mildlycontext-sensitive languages.

A reversible automaton is a finite automaton in which eachletter induces a partial one-to-one map from the set of states intoitself. We solve the following problem proposed by Pin. Given an alphabet A, does there exist a sequence of languages K n on A which can be accepted by a reversible automaton, and such that the number of states of the minimal automaton of K n is in O(n), while the minimal number of states of a reversible automaton accepting K n is in O(ρ n ) for some ρ > 1? We give such an example with $\rho=\left(\frac{9}{8}\right)^{\frac{1}{12}}$ .