Finite biprefix sets of paths in a graph

AbstractThe main results of the combinatorial theory of maximal biprefix codes of words (Césari, Perrin, Schützenberger) are extended to the codes of paths in a graph in this paper: degree and decoding of double-infinite paths, finiteness of codes of a given degree, the Césari-Schützenberger algorithm, derivation and integration of codes will be discussed

Characteristic morphisms of generalized episturmian words

In a recent paper with L. Q. Zamboni, the authors introduced the class of ϑ-episturmian words. An infinite word over A is standard ϑ-episturmian, where ϑ is an involutory antimorphism of A*, if its set of factors is closed under ϑ and its left special factors are prefixes. When ϑ is the reversal operator, one obtains the usual standard episturmian words. In this paper, we introduce and study ϑ-characteristic morphisms, that is, morphisms which map standard episturmian words into standard ϑ-episturmian words. They are a natural extension of standard episturmian morphisms. The main result of the paper is a characterization of these morphisms when they are injective. In order to prove this result, we also introduce and study a class of biprefix codes which are overlap-free, i.e., any two code words do not overlap properly, and normal, i.e., no proper suffix (prefix) of any code-word is left (right) special in the code. A further result is that any standard ϑ-episturmian word is a morphic image, by an injective ϑ-characteristic morphism, of a standard episturmian word

On Commutation and Conjugacy of Rational Languages and the Fixed Point Method

The research on language equations has been active during last decades. Compared to the equations on words the equations on languages are much more difficult to solve. Even very simple equations that are easy to solve for words can be very hard for languages. In this thesis we study two of such equations, namely commutation and conjugacy equations. We study these equations on some limited special cases and compare some of these results to the solutions of corresponding equations on words. For both equations we study the maximal solutions, the centralizer and the conjugator. We present a fixed point method that we can use to search these maximal solutions and analyze the reasons why this method is not successful for all languages. We give also several examples to illustrate the behaviour of this method.Siirretty Doriast

A hierarchy of irreducible sofic shifts

International audienceWe define new subclasses of the class of irreducible sofic shifts. These classes form an infinite hierarchy where the lowest class is the class of almost finite type shifts introduced by B. Marcus. We give effective characterizations of these classes with the syntactic semigroups of the shifts

Systems of equations over a free monoid and Ehrenfeucht's conjecture

AbstractEhrenfeucht's conjecture states that every language L has a finite subset F such that, for any pair (g, h) of morphisms, g and h agree on every word of L if and only if they agree on every word of F. We show that it holds if and only if every infinite system of equations (with a finite number of unknowns) over a free monoid has an equivalent finite subsystem. It is shown that this holds true for rational (regular) systems of equations.The equivalence and inclusion problems for finite and rational systems of equations are shown to be decidable and, consequently, the validity of Ehrenfeucht's conjecture implies the decidability of the HDOL and DTOL sequence equivalence problems. The simplicity degree of a language is introduced and used to argue in support of Ehrenfeucht's conjecture

Condons and Codes

In this paper we assemble a few ingredients that are remotely connected to each other, but governed by the rule of coding theory ([1], [12]) and formal language theory, i.e. cyclic codes and DNA codes. Our interest arose from the remark that there exist both linear and circular DNAs in higher living organisms. We state the theory of codes in a wide sense due to [1] in order to understand the circular DNAs while we state rudiments of formal language theory as a means to interpret genes. We hope this will be a starter for unifying two approaches depending on the theory of codes and that of formal language

On almost cylindrical languages and the decidability of the D0L and PWD0L primitivity problems

AbstractPrimitive words and their properties have always been of fundamental importance in the study of formal language theory. Head and Lando in Periodic D0L Languages proposed the idea of deciding whether or not a given D0L language has the property that every word in it is a primitive word. After reducing the general problem to the case in which h is injective, it will be shown that primitivity is decidable when ((A)h)∗ is an almost cylindrical set. Moreover, in this case, it is shown that the set of words which generate primitive sequences (given a particular D0L scheme) is an algorithmically constructible context-sensitive language. An undecidability result for the PWD0L primitivity problem and decidability results for cases of the RWD0L primitivity problem are also given

Growth of Algebras and Codes

This dissertation is devoted to the study of the growth of algebras and formal languages. It consists of three parts. The first part is devoted to the growth of finitely presented quadratic algebras. The study of these algebras was motivated by the question about the growth types of Koszul algebras which are a special subclass of finitely presented quadratic algebras. We show that there exist finitely presented quadratic algebras of intermediate growth and give two concrete examples of such algebras with their presentations. The second part focuses on the study of the growth of metabelian Lie algebras and their universal enveloping algebras. Our motivation was to construct finitely presented algebras of different intermediate growth types. As an outcome of this investigation we prove that for any d 2 N there exists a finitely presented algebra whose growth function is equivalent to e^n^d=(d+1). The last part focuses on infinite codes over finite alphabets, their properties and growth. A special attention is paid to S-codes, weak S-codes and Markov codes which play an important role in coding theory and ergodic theory. We investigate what types of codes may have maximal growth. Also, we prove that S-codes covering Bernoulli schemes are maximal