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Star-Connected Flat Languages and Automata

Klunder B.

Fundamenta Informaticae

2006

Vol. 72, nr 1-3

235-243

Star-connected rational expressions were considered only as expressions defining trace languages. In this paper we continue the study of the class of flat languages defined by this kind of rational expressions. We strengthen results of [4] and prove that any star-connected language has an automaton with cycles which are composition of connected cycles. This condition is sufficient for star-connected languages provided that the dependency relation is transitive. As a consequence we obtain for every alphabet stronger pumping lemma for star-connected languages. Finally, the product of two automata inherits this property from the components and thus for alphabet with transitive dependency relation the set of all star-connected languages is closed under intersections.

On Star-Connected Flat Languages

Klunder B., Ochmański E., Stawikowska K.

Fundamenta Informaticae

2005

Vol. 67, nr 1-3

93--105

Up to now, star-connected rational expressions were considered only as expressions defining trace languages. In this paper we study the class of flat languages defined by this kind of rational expressions. We consider regular languages inducing recognizable trace languages and its subclasses: languages of finite ranks and star-connected languages. Main results are the following: rank of any star-connected language is finite; any star-connected language has an automaton with connected simple cycles; two pumping lemmas for star-connected languages. Moreover, we show that none of the investigated classes is closed under complement and that the class of languages of finite ranks is not closed under intersection. Finally, we briefly mention some decision problems and formulate several open questions.