We study two encodings of the asynchronous #-calculus with input-guarded choice into its choice-free fragment. One encoding is divergence-free, but refines the atomic commitment of choice into gradual commitment. The other preserves atomicity, but introduces divergence. The divergent encoding is fully abstract with respect to weak bisimulation, but the more natural divergence-free encoding is not. Instead, we show that it is fully abstract with respect to coupled simulation, a slightly coarser---but still coinductively defined---equivalence that does not enforce bisimilarity of internal branching decisions. The correctness proofs for the two choice encodings introduce a novel proof technique exploiting the properties of explicit decodings from translations to source terms.

We extend the π-calculus with polyadic synchronisation, a generalisation of the communication mechanism which allows channel names to be composite. We show that this operator embeds nicely in the theory of π-calculus, we suggest that it permits divergence-free encodings of distributed calculi, and we show that a limited form of polyadic synchronisation can be encoded weakly in π-calculus. After showing that matching cannot be derived in π-calculus, we compare the expressivity of polyadic synchronisation, mixed choice and matching. In particular we show that the degree of synchronisation of a language increases its expressive power by means of a separation result in the style of Palamidessi's result for mixed choice.

We present a calculus of mobile processes without prefix or summation, and using two different encodings we show that it can express both action prefix and guarded summation. One encoding gives a strong correspondence but uses a match operator; the other yields a slightly weaker correspondence but uses no additional operators.

) y Nobuko Yoshida ? Abstract. In [18, 19], we presented a theory of concurrent combinators for the asynchronous monadic ß-calculus without match or summation operator [7, 16]. The system of concurrent combinators is based on a finite number of atoms and fixed interaction rules, but is as expressive as the original calculus, so that it can represent diverse interaction structures, including polyadic synchronous name passing [23] and input guarded summations [26]. The present paper shows that each of the five basic combinators introduced in [18] is indispensable to represent the whole computation, i.e. if one of the combinators is missing, we can no longer express the original calculus up to weak bisimilarity. Expressive power of several interesting subsystems of the asynchronous ß-calculus is also measured by using appropriate subsets of the combinators and their variants. Finally as an application of the main result, we show there is no semantically sound encoding of the calculus in...

Abstract. This paper compares the expressiveness of ambient calculi against different dialects of the pi-calculus. Cardelli and Gordon encoded the asynchronous pi-calculus into their calculus of Mobile Ambients (MA). Zimmer has shown that the synchronous pi-calculus without choice can be encoded in pure (no communication) Safe Ambients. We show that pure MA without restriction has symmetric electoral systems, that is, it is possible to solve the problem of electing a leader in a symmetric network. By the work of Palamidessi, this implies that pure MA without restriction is not encodable (under certain conditions) in the picalculus with separate choice. We adapt the work of Carbone and Maffeis to show that pure MA cannot be encoded (under certain other conditions) into the pi-calculus with mixed choice (but without matching). 1

The join-calculus was introduced as an `extended subset' of the asynchronous π-calculus by amalgamating the three operators for input, restriction, and replication into a single operator, called definition, but with the additional capability to describe the atomic joint reception of values from two different channels. In this paper, we just extend the asynchronous π-calculus with joint input. By studying its expressive power, using slight variations of previously investigated choice encodings, we also conclude on the expressiveness of the join-calculus.

Abstract. A term terminates if all its reduction sequences are of finite length. We show four type systems that ensure termination of well-typed ss-calculus processes. The systems are obtained by successive refinements of the types of the simply typed ss-calculus. For all (but one of) the type systems we also present upper bounds to the number of steps well-typed processes take to terminate. The termination proofs use techniques from term rewriting systems. We show the usefulness of the type systems on some non-trivial examples: the encodings of primitive recursive functions, the protocol for encoding separate choice in terms of parallel composition, a symbol table implemented as a dynamic chain of cells. 1 Introduction A term terminates if all its reduction sequences are of finite length. As far as programminglanguages are concerned, termination means that computation in programs will eventually stop. In computer science termination has been extensively investigated in term rewritingsystems [7, 5] and *-calculi [9, 4] (where strong normalization is a synonym more commonlyused). Termination has also been discussed in process calculi, notably the

We consider a generalization of the dining philosophers problem to arbitrary connection topologies. We focus on symmetric, fully distributed systems, and we address the problem of guaranteeing progress and lockout-freedom, even in presence of adversary schedulers, by using randomized algorithms. We show that the well-known algorithms of Lehmann and Rabin do not work in the generalized case, and we propose an alternative algorithm based on the idea of letting the philosophers assign a random priority to their adjacent forks.

First of all, I would like to thank my supervisor Dr. Iain C. C. Phillips, for his support and collaboration during this period of research. I thank Iain for having taught me to be more precise and sharp, and for long, detailed and inspiring discussions on the topic of this dissertation. Finally I thank him for his enormous patience towards my stubbornness. I would like to thank Dr. Nobuko Yoshida for many useful discussions and for being very supportive and positive about my work. To Sergio Maffeis go thanks for many discussions on various subjects of research and philosophy during the last two years at Imperial College. He suggested an improvement to the solution for the leader election problem for the Ambient Calculus. I would like to thank also Andrew Phillips, and the concurrency group at Imperial for the Monday lunch meetings. This has been a wonderful forum for discussing various aspects of my work. I like to thank Prof. Chris Hankin and Dr. Sophia Drossopoulou for helping me on various occasions with administrative problems and (especially Chris) for supporting most of my travelling. I do not know how I could have ever achieved this without my husband, Steffen van Bakel. He

We present an expressiveness study of linearity and persistence of processes. We choose the π-calculus, one of the main representatives of process calculi, as a framework to conduct our study. We consider four fragments of the π-calculus. Each one singles out a natural source of linearity/persistence also present in other frameworks such as Concurrent Constraint Programming (CCP), Linear CCP, and several calculi for security. The study is presented by providing (or proving the non-existence of) encodings among the fragments, a processes-as-formulae interpretation and a reduction from Minsky machines. 1