Thin and thick timed regular languages

Abstract

In previous literature on timed automata, it was noticed that they are in several aspects too precise, which leads sometimes to strange artifacts, mathematical pathologies or unrealistic models. In particular, some timed automata are non-implementable, non-robust, behave badly under discretization, have many Zeno runs etc. In this paper, we propose a unifying approach to most of these issues for deterministic timed automata. We classify these automata either as thin or as thick. In thin automata, all the infinite trajectories are, in some weak sense, Zeno; the discretization of long trajectories is difficult, since it requires very small discretization step. In thick automata, most of trajectories are non-Zeno and behave well under discretization; such automata satisfy a sort of pumping lemma. Formally, the thin-thick alternative is based on the notion of entropy of timed regular languages introduced by E. Asarin and A. Degorre in [3,4]. Thin languages have the entropy = -∞ while thick have a larger one. An important application of thin-thick alternative is again the entropy theory of timed languages. We show that the entropy can be computed with a desired precision using discretization and thus it is computable, which closes a question left open in [3,4]. © 2011 Springer-Verlag.