Real-Time Logics: Complexity and Expressiveness

Rajeev Alur and Thomas A. Henzinger

The theory of the natural numbers with linear order and monadic predicates underlies propositional linear temporal logic. To study temporal logics that are suitable for reasoning about real-time systems, we combine this classical theory of infinite state sequences with a theory of discrete time, via a monotonic function that maps every state to its time. The resulting theory of timed state sequences is shown to be decidable, albeit nonelementary, and its expressive power is characterized by omega-regular sets. Several more expressive variants are proved to be highly undecidable.

This framework allows us to classify a wide variety of real-time logics according to their complexity and expressiveness. Indeed, it follows that most formalisms proposed in the literature cannot be decided. We are, however, able to identify two elementary real-time temporal logics as expressively complete fragments of the theory of timed state sequences, and we present tableau-based decision procedures for checking validity. Consequently, these two formalisms are well-suited for the specification and verification of real-time systems.

Information and Computation 104:35-77, 1993. A preliminary version appeared in the Proceedings of the Fifth Annual Symposium on Logic in Computer Science (LICS), IEEE Computer Society Press, 1990, pp. 390-401.

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