We present an actor language which is an extension of a simple functional language, and provide a precise operational semantics for this extension. Actor configurations represent open distributed systems, by which we mean that the specification of an actor system explicitly takes into account the interface with external components. We study the composability of such systems. We define and study various notions of testing equivalence on actor expressions and configurations. The model we develop provides fairness. An important result is that the three forms of equivalence, namely, convex, must, and may equivalences, collapse to two in the presence of fairness. We further develop methods for proving laws of equivalence and provide example proofs to illustrate our methodology.

Finite automata are the basis of many verification methods and tools for process algebras. It is however undecidable in most process algebras whether the semantics of a given term is finite. We give sufficient finiteness conditions derived from the analysis of the operational rules of the algebra operators. From these rules we also generate the functions that compute automata from terms of the algebra. These constructions allow one to use our verification tools for programs written in many process algebras. Keywords verification, concurrent systems, process algebra, lotos, structural operational semantics, finite automata. 1 Introduction Verification methods for concurrent systems can be classified in at least three families: theorem proving methods, model-checking, and automata based methods. The first family holds the biggest theoretical power; it may be applied to many sort of undecidable problems and in some sense it can deal with infinite objects. However theorem proving m...