Abstract

The expressive power of process algebras is investigated in a general setting of structural operational semantics. The notion of an effective operational semantics is introduced and it is observed that no effective operational semantics for an enumerable language can specify all effective process graphs up to trace equivalence. A natural class of Plotkin style SOS specifications is identified, containing the guarded versions of calculi like CCS, SCCS, Meije and ACP, and it is proved that any specification in this class induces an effective 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 final result of this paper is that all operators definable via a finite number of rules in a format due to De Simone, are derived operators in the simple process calculus PC. 1991 Mathematics Subject Classification: 68Q05, 68Q10, 68Q55, 68Q75, 03D20. 1991 CR Categories: D.3.1, D.3.3, F.1.1, F.1.2, F.3.2, F.4.1. Keywords & Phrases: process algebra, PC, labeled transition systems, process graphs, effective process graphs, effective operational semantics, structural operational semantics, expressiveness, bisimulation equivalence, trace equivalence, action transducers. Notes: Most of this work was carried out while the author was at the MIT Laboratory for Computer Science, supported by ONR contract N00014-85-K-0168. Part of this work took place in the context of the ESPRIT Basic Research Action 7166, CONCUR2. This p...

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