Beyond ωBS-regular Languages: ωT-regular Expressions and Counter-Check Automata

Dario Della Monica
(Universidad Complutense de Madrid, Spain, and Università "Federico II" di Napoli, Italy)
Angelo Montanari
(Università di Udine, Italy)
Pietro Sala
(Università di Verona)

In the last years, various extensions of ω-regular languages have been proposed in the literature, including ωB-regular (ω-regular languages extended with boundedness), ωS-regular (ω-regular languages extended with strict unboundedness), and ωBS-regular languages (the combination of ωB- and ωS-regular ones). While the first two classes satisfy a generalized closure property, namely, the complement of an ωB-regular (resp., ωS-regular) language is an ωS-regular (resp., ωB-regular) one, the last class is not closed under complementation. The existence of non-ωBS-regular languages that are the complements of some ωBS-regular ones and express fairly natural properties of reactive systems motivates the search for other well-behaved classes of extended ω-regular languages. In this paper, we introduce the class of ωT-regular languages, that includes meaningful languages which are not ωBS-regular. We first define it in terms of ωT-regular expressions. Then, we introduce a new class of automata (counter-check automata) and we prove that (i) their emptiness problem is decidable in PTIME and (ii) they are expressive enough to capture ωT-regular languages (whether or not ωT-regular languages are expressively complete with respect to counter-check automata is still an open problem). Finally, we provide an encoding of ωT-regular expressions into S1S+U.

In Patricia Bouyer, Andrea Orlandini and Pierluigi San Pietro: Proceedings Eighth International Symposium on Games, Automata, Logics and Formal Verification (GandALF 2017), Roma, Italy, 20-22 September 2017, Electronic Proceedings in Theoretical Computer Science 256, pp. 223–237.
Published: 6th September 2017.

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