RAPPORT Expressiveness results for process

The expressive power of process algebras is investigated in a general setting of structural operational semantics. The notion of an eeective operational semantics is introduced and it is observed that no eeective operational semantics for an enumerable language can specify all eeective process graphs up to trace equivalence. A natural class of Plotkin style SOS speciications is identiied, containing the guarded versions of calculi like CCS, SCCS, Meije and ACP, and it is proved that any speciication in this class induces an eeective operational semantics. Using techniques introduced by Bloom, it is shown that for the guarded versions of CCS-like calculi, there is a double exponential bound on the speed with which the number of outgoing transitions in a state can grow. As a corollary of this result it follows that two expressiveness results of De Simone for Meije and SCCS depend in a fundamental way on the use of unguarded recursion. A nal result of this paper is that all operators deenable via a nite number of rules in a format due to De Simone, are derived operators in the simple process calculus PC.