Axiomatizing Probabilistic Processes: ACP with Generative Probabilities

AbstractThis paper is concerned with finding complete axiomatizations of probabilistic processes. We examine this problem within the context of the process algebra ACP and obtain as our endresult the axiom system pr ACP−l, a version of ACP whose main innovation is a probabilistic asynchronous interleaving operator. Our goal was to introduce probability into ACP in as simple a fashion as possible, Optimally, ACP should be the homomorphic image of the probabilistic version in which the probabilities are forgotten, We begin by weakening slightly ACP to obtain the axiom system ACP−l. The main difference between ACP and ACP−l is that the axiom x + δ = x, which does not yield a plausible interpretation in the generative model of probabilistic computation, is rejected in ACP−l. We argue that this does not affect the usefulness of ACP−l in practice, and show how ACP can be reconstructed from ACP−l with a minimal amount of technical machinery. pr ACP−l is obtained from ACP−l through the introduction of probabilistic alternative and parallel composition operators, and a process graph model for pr ACP−l based on probabilistic bisimulation is developed. We show that pr ACP−l is a sound and complete axiomatization of probabilistic bisimulation for finite processes, and that pr ACP−l can be homomorphically embedded in ACP−l as desired. Our results for ACP−l and pr ACP−l are presented in a modular fashion by first considering several subsets of the signatures, We conclude with a discussion about adding an iteration operator to pr ACP−l