this paper have been presented at the 4th European Summer School in Logic, Language and Information, University of Essex, 1992; at the Tempus Summer School for Algebraic and Categorical Methods in Computer Science, Masaryk University, Brno, 1993; and the Summer School in Logic Methods in Concurrency, Aarhus University, 1993. I would like to thank the organisers and the participants of these summer schools, and of the Banff higher order workshop. I would also like to thank Julian Bradfield for use of his Tex tree constructor for building derivation trees and Carron Kirkwood, Faron Moller, Perdita Stevens and David Walker for comments on earlier drafts.

Baeten, Bergstra, and Klop (and later Caucal) have proved the remarkable result that bisimulation equivalence is decidable for irredundant context-free grammars. In this paper we provide a much simpler and much more direct proof of this result using a tableau decision method involving goal-directed rules. The decision procedure also provides the essential part of the bisimulation relation between two processes which underlies their equivalence. We also show how to obtain a sound and complete sequent-based equational theory for such processes from the tableau system and how one can extract what Caucal calls a fundamental relation from a successful tableau.

We show that characteristic formulae for nite-state systems up to bisimulation-like equivalences (e.g., strong and weak bisimilarity) can be given in the simple branching-time temporal logic EF. Since EF is a very weak fragment of the modal µ-calculus, model checking with EF is decidable for many more classes of infinite-state systems. This yields a general method for proving decidability of bisimulation-like equivalences between infinite-state processes and finite-state ones. We apply this method to the class of PAD processes, which strictly subsumes PA and pushdown (PDA) processes, showing that a large class of bisimulation-like equivalences (including, e.g., strong and weak bisimilarity) is decidable between PAD and finite-state processes. On the other hand, we also demonstrate that no `reasonable' bisimulation-like equivalence is decidable between state-extended PA processes and finite-state ones. Furthermore, weak bisimilarity with finite-state processes is shown to be undecidable even for state-...

. Recent results show that strong bisimilarity is decidable for the class of Basic Parallel Processes (BPP), which corresponds to the subset of CCS definable using recursion, action prefixing, nondeterminism and the full merge operator. In this paper we examine all other equivalences in the linear/branching time hierarchy [12] and show that none of them are decidable for BPP. 1 Introduction Much attention has been devoted to the study of process calculi and in particular to behavioural semantics for these calculi. In order to capture the behavioural aspects of processes, a variety of equivalences have been proposed. Various criteria exist for comparing the merits and deficiencies of these equivalences. A systematic approach consists of classifying the equivalences according to their coarseness. For this purpose van Glabbeek proposed the linear/branching time spectrum which is illustrated in Figure 1 [12]. The least discriminating equivalences are at the bottom of the diagram. Arrows i...

A process # is regular if it is bisimilar to a process # # with finitely many states. We prove that regularity of normed PA processes is decidable and we present a practically usable polynomial-time algorithm. Moreover, if the tested normed PA process # is regular then the process # # can be e#ectively constructed. It implies decidability of bisimulation equivalence for any pair of processes such that one process of this pair is a normed PA process and the other process has finitely many states.

this paper we survey recent developments and current trends within a new field of study within process algebra, namely that of decidability issues for processes with infinite transition graphs.

We consider the problem of simulation preorder/equivalence between infinitestate processes and finite-state ones. We prove that simulation preorder (in both directions) and simulation equivalence are intractable between all major classes of infinite-state systems and finite-state ones. This result is obtained by showing that the problem whether a BPA (or BPP) process simulates a finite-state one is PSPACE-hard, and the other direction is co-NP-hard; consequently, simulation equivalence between BPA (or BPP) and finite-state processes is also co-NP-hard. The decidability border for the mentioned problem is also established. Simulation preorder (in both directions) and simulation equivalence are shown to be decidable in EXPTIME between pushdown processes and finite-state ones. On the other hand, simulation preorder is undecidable between PA and finite-state processes in both directions. The obtained results also hold for those PA and finite-state processes which are determini...

Abstract. We prove that weak bisimilarity is decidable in polynomial time between BPA and finite-state processes, and between normed BPP and finite-state processes. To the best of our knowledge, these are the first polynomial algorithms for weak bisimilarity with infinite-state systems. 1

Program transformation is a technique for obtaining, starting from a program P, a semantically equivalent one, which is "better" than P with respect to a particular goal. Traditionally, the main goal of program transformation was obtaining more efficient programs, but, in general, this technique can be used to produce programs written in a syntactic form satisfying some properties. Program transformation techniques have been extensively studied in the framework of functional and logic languages, where they were applied mainly to obtain more efficient and readable programs. All these works are based on the Unfold/Fold program transformation method developed by Burstall and Darlington in the context of their recursive equational language. The use of Unfold /Fold based transformations for concurrent languages is a relevant issue that has not yet received an adequate attention. In fact the existing proposals of transformations of concurrent programs are not based on a general Unfold/Fold transformation theory. The aim of this paper is to define such a theory for the concurrent calculus CCS and to prove it correct. 1

Abstract. We consider the problem of deciding regularity of normed BPP and normed BPA processes. A process is regular if it is bisimilar to a process with finitely many states. We show that regularity of normed BPP and normed BPA processes is decidable in polynomial time and we present constructive regularity tests. Combining these two results we obtain a rich subclass of normed PA processes (called sPA) where the regularity is also decidable. Moreover, constructiveness of this result implies decidability of bisimilarity for pairs of processes such that one process of this pair is sPA and the other has finitely many states. 1