265 research outputs found
Subproduct systems and Cartesian systems; new results on factorial languages and their relations with other areas
We point out that a sequence of natural numbers is the dimension sequence of
a subproduct system if and only if it is the cardinality sequence of a word
system (or factorial language). Determining such sequences is, therefore,
reduced to a purely combinatorial problem in the combinatorics of words. A
corresponding (and equivalent) result for graded algebras has been known in
abstract algebra, but this connection with pure combinatorics has not yet been
noticed by the product systems community. We also introduce Cartesian systems,
which can be seen either as a set theoretic version of subproduct systems or an
abstract version of word systems. Applying this, we provide several new results
on the cardinality sequences of word systems and the dimension sequences of
subproduct systems.Comment: New title; added references; to appear in Journal of Stochastic
Analysi
Profinite Groups Associated to Sofic Shifts are Free
We show that the maximal subgroup of the free profinite semigroup associated
by Almeida to an irreducible sofic shift is a free profinite group,
generalizing an earlier result of the second author for the case of the full
shift (whose corresponding maximal subgroup is the maximal subgroup of the
minimal ideal). A corresponding result is proved for certain relatively free
profinite semigroups. We also establish some other analogies between the kernel
of the free profinite semigroup and the \J-class associated to an irreducible
sofic shift
Some structural properties of the free profinite aperiodic semigroup
Profinite semigroups provide powerful tools to understand properties of classes of regular languages. Until very recently however, little was known on the structure of "large" relatively free profinite semi- groups. In this paper, we present new results obtained for the class of all finite aperiodic (that is, group-free) semigroups. Given a finite alphabet X, we focus on the following problems: (1) the word problem for ω-terms on X evaluated on the free pro-aperiodic semigroup, and (2) the computation of closures of regular languages in the ω-subsemigroup of the free pro-aperiodic semigroup generated by X.FCT through the Centro de Matemática da Universidade do Minho and the Centro de Matemática da Universidade do PortoEuropean Community Fund FEDERESF programme “Automata: from Mathematics to Applications (AutoMathA)”Pessoa Portuguese-French project Egide-Grices 11113Y
Factoriality and the pin-reutenauer procedure
We consider implicit signatures over finite semigroups determined by sets of pseudonatural numbers. We prove that, under relatively simple hypotheses on a pseudovariety V of semigroups, the finitely generated free algebra for the largest such signature is closed under taking factors within the free pro-V semigroup on the same set of generators. Furthermore, we show that the natural analogue of the Pin-Reutenauer descriptive procedure for the closure of a rational language in the free group with respect to the profinite topology holds for the pseudovariety of all finite semigroups. As an application, we establish that a pseudovariety enjoys this property if and only if it is full.European structural funds (FEDER)European Regional Development Fund, through the program COMPET
Iterated periodicity over finite aperiodic semigroups
This paper provides a characterization of pseudowords over the pseudovariety of all finite aperiodic semigroups that can be described from the free generators using only the operations of multiplication and omega-power. A necessary and sufficient condition for this property to hold turns out to be given by the conjunction of two rather simple finiteness conditions: the nonexistence of infinite anti-chains of factors and the rationality of the language of McCammond normal forms of omega-terms that define factors of the given pseudoword. The relationship between pseudowords with this property and arbitrary pseudowords is also investigated.Projeto PTDC/MAT/65481/2006 financiado em parte pelo European Community
Fund FEDERPrograma AutoMathA da European Science Fundation (ESF)Projeto Egide-Grices 11113YMFundação para a Ciência e a Tecnologia (FCT
Cost and dimension of words of zero topological entropy
Let denote the free monoid generated by a finite nonempty set In
this paper we introduce a new measure of complexity of languages defined in terms of the semigroup structure on For each we define its {\it cost} as the infimum of all real numbers
for which there exist a language with
and a positive integer with We also
define the {\it cost dimension} as the infimum of the set of all
positive integers such that for some language with
We are primarily interested in languages given by the
set of factors of an infinite word of zero
topological entropy, in which case We establish the following
characterisation of words of linear factor complexity: Let and
Fac be the set of factors of Then if and only
and In other words, if and only if
Fac for some language of bounded complexity
(meaning In general the cost of a language
reflects deeply the underlying combinatorial structure induced by the semigroup
structure on For example, in contrast to the above characterisation of
languages generated by words of sub-linear complexity, there exist non
factorial languages of complexity (and hence of cost
equal to and of cost dimension In this paper we investigate the
cost and cost dimension of languages defined by infinite words of zero
topological entropy
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