STAR OPERATORS ON sn-NETWORKS

We introduce an operator on classes of regular languages, the star-free closure. Our motivation is to generalize standard results of automata theory within a unified framework. Given an arbitrary input class \(\mathscr{C}\) , the star-free closure operator outputs the least class closed under Boolean operations and language concatenation, and containing all languages of \(\mathscr{C}\) as well as all finite languages. We establish several equivalent characterizations of star-free closure: in terms of regular expressions, first-order logic, pure future and future-past temporal logic, and recognition by finite monoids. A key ingredient is that star-free closure coincides with another closure operator, defined in terms of regular operations where Kleene stars are allowed in restricted contexts. A consequence of this first result is that we can decide membership of a regular language in the star-free closure of a class whose separation problem is decidable. Moreover, we prove that separation itself is decidable for the star-free closure of any finite class, and of any class of group languages having itself decidable separation (plus mild additional properties). We actually show decidability of a stronger property, called covering.

Multiplicative closure operations on ring extensions

A famous theorem of Kuratowski states that, in a topological space, at most 14 distinct sets can be produced by repeatedly applying the operations of closure and complement to a given set. We re-examine this theorem in the setting of formal languages, where closure is either Kleene closure or positive closure. We classify languages according to the structure of the algebra they generate under iterations of complement and closure. There are precisely 9 such algebras in the case of positive closure, and 12 in the case of Kleene closure. We study how the properties of being open and closed are preserved under concatenation. We investigate analogues, in formal languages, of the separation axioms in topological spaces; one of our main results is that there is a clopen partition separating two words if and only if the words do not commute. We can decide in quadratic time if the language specified by a DFA is closed, but if the language is specified by an NFA, the problem is PSPACE-complete.KeywordsTopological SpaceFormal LanguageClosure OperatorRegular LanguageShort WordThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Union-free expressions are used in union normal form to decompose any regular language to a finite union of union-free languages. Based on the automata characterisation of the union-free languages, by restricting the 1CFPAs not to have transitions by the empty word, or to be deterministic, the n-union-free and the deterministic union-free languages are defined. Union-complexity as a measure of descriptional complexity of regular languages was introduced recently. By the minimum number of union-free/n-union-free/deterministic union-free languages needed to get a regular language as their union, its union-complexity/n-union-complexity/d-union-complexity is defined. It is already known that union-complexity and n-union-complexity are finite for every regular language, however there are regular languages with infinite d-union-complexity. Operational union-complexity, that is, to predict the union-complexity of a language obtained by a language operation from languages with known union-complexity is an important and interesting question belonging to the field of descriptional complexity of formal systems. In the present paper, the Kleene plus, the positive Kleene closure operator is studied. As the Kleene star and plus operations have very different effects on the union-free languages, it is an interesting problem to investigate how the union-complexities may change under this operation. In particular, we show that the union-complexity of a regular language is not growing when this operation is being applied on it. On the other hand, the n-union-complexity of the Kleene plus of an n-union-free language remains 1, but the n-union-complexity of the Kleene plus of other regular languages may grow. Further, the deterministic union-complexity may jump to an infinite value even if the original language had a relatively small deterministic union-complexity, e.g., 4.Keywordsunion-complexityunion-free languagesregular expressionsKleene closure

In this study, we introduce the concept of lattice-valued regular expressions. Such expressions become not only the necessary tool for the analysis and synthesis of fuzzy automata, but give rise to a recursive generation of the family of fuzzy languages accepted by fuzzy automata from certain simple fuzzy languages. The equivalence between lattice-valued regular expressions and lattice-valued finite automata is demonstrated. We also show that the family of fuzzy languages accepted by deterministic lattice-valued finite automata (abbreviated as DLA) is not closed under Kleene closure operation. Similarly, we show that in general the family of fuzzy languages accepted by (nondeterministic) lattice-valued finite automata (abbreviated as LA) is not closed under complement operation. These two findings form an essential difference between lattice-valued finite automata and classical finite automata. We also elaborate on regular expressions produced by DLA.