We study syntax-free models for name-passing processes. For interleaving semantics, we identify the indexing structure required of an early labelled transition system to support the usual pi-calculus operations, defining Indexed Labelled Transition Systems. For noninterleaving causal semantics we define Indexed Labelled Asynchronous Transition Systems, smoothly generalizing both our interleaving model and the standard Asynchronous Transition Systems model for CCS-like calculi. In each case we relate a denotational semantics to an operational view, for bisimulation and causal bisimulation respectively. We establish completeness properties of, and adjunctions between, categories of the two models. Alternative indexing structures and possible applications are also discussed. These are first steps towards a uniform understanding of the semantics and operations of name-passing calculi.

Abstract. One obtains in this paper a process algebra RCCS, in the style of CCS, where processes can backtrack. Backtrack, just as plain forward computation, is seen as a synchronization and incurs no additional cost on the communication structure. It is shown that, given a past, a computation step can be taken back if and only if it leads to a causally equivalent past. 1

Abstract. We propose a formalisation of the notion of transaction, using a variant of CCS, RCCS, that distinguishes reversible and irreversible actions, and incorporates a distributed backtrack mechanism. Any weakly correct implementation of a given transaction in CCS, once embedded in RCCS, automatically obtains a correct one. We show examples where this method allows for a more concise implementation and a simpler proof of correctness. 1

We present CCS-R, a reversible variant of Milner's CCS. This simple process algebra o#ers a backtracking mechanism which is shown to be sound. We then discuss how biological systems satisfying a "perfect mix" assumption can be formalized within CCS-R.

We consider (a slight variant of) the ccs calculus, and we analyze two operational semantics defined in the literature: the first exploits Proved Transition Systems (pts) and the second Rewriting Logic (rl). We show that the interleaving interpretation of both semantics agree, in that they define the same transitions and exhibit the same nondeterministic structure. In addition, we study causality in ccs computations. We recall the treatment via pts, that exhibits the notion of causality presented in the literature, and we show how to recast it in the rl semantics via suitable axioms. 1 Introduction Concurrency is maybe the basic aspect of the operational interpretation of rewriting logic. And as Jos'e Meseguer says in his lecture at concur'96 [20], . . . my main emphasis in this talk will be on rewriting logic as a semantic framework for concurrency. . . . The goal is . . . to express as faithfully as possible each model [of concurrency] on its own terms, avoiding any encodings or tr...

We consider two operational semantics for ccs dened in the literature: the rst exploits Proved Transition Systems (pts) and the second Rewriting Logic (rl). We show that the interleaving interpretation of both semantics agree, in that they dene the same transitions and exhibit the same nondeterministic structure. In addition, we study causality in ccs computations. We recall its treatment via pts, exhibiting the notion of causality presented in the literature, and we show how to recast it in the rl semantics via suitable axioms. Also in this case, the two semantics agree. Contents 1 Introduction 2 2 Some notions on Process Algebras 3 2.1 The Calculus of Communicating Systems 4 2.2 Proved Transition System 6 2.3 Causality and Concurrency 7 ? Research partly supported by the Italian CNR Progetto Strategico Modelli e Metodi per la Matematica e l'Ingegneria and MURST Progetto Tecniche Formali per la Specica, l'Analisi, la Verica, la Sintesi e la Trasformazione di Sistemi Software. ...

Summary. P systems are a biologically inspired model introduced by Gheorghe Păun with the aim of representing the structure and the functioning of the cell. P systems are usually equipped with the maximal parallelism semantics; however, since their introduction, some alternative semantics have been proposed and investigated. We propose a semantics that describes the causal dependencies occurring between the reactions of a P system. We investigate the basic properties that are satisfied by such a semantics. The notion of causality turns out to be quite relevant for biological systems, as it permits to point out which events occurring in a biological pathway are necessary for another event to happen. 1

We extend the #-calculus and the spi-calculus with two primitives that guarantee authentication.

In synchronous programming, programs can be perceived as purely sequential, and parallelism is only a logical concept useful to develop programs in a modular way. Classical semantics for synchronous languages interpret programs as sequential input/output finite state machines (i/o FSMs) where concurrency disappears. We argue that semantics where concurrency is reected can be useful at least for improving hardware implementation, veri cation, and design of model-based schedulers. So, these semantics should not "compete" with the classical ones but should o er di erent, although consistent, views of programs, each supporting a particular task in their development. In order to de ne semantics reecting concurrency, well established techniques adopted to define "truly concurrent" semantics for asynchronous languages could be applied. In this paper, we consider as a prototype of the class of synchronous languages the language Esterel, which is gaining use in a wide variety of applications. Then, following a method proposed by Degano and Priami to give semantics for asynchronous process algebras, we develop a semantic framework in which one can define different, although consistent, semantics of Esterel programs. The idea is to define a very concrete model of the language from which more abstract models can be recovered by means of suitable abstraction functions. We define a proved transition system (PTS) as the most concrete model of Esterel. We show that the classical interpretation in FSMs can be recovered from the PTS by a suitable abstraction function, namely we show that our most concrete semantics is consistent with the classical one. Then, from proved trees obtained by unfolding parts of the PTS, we abstract locality trees and enabling trees. We show how locality trees c...

Abstract. Event structures have been used for giving true concurrent semantics to languages and models of concurrency such as CCS, Petri nets and graph grammars. Although certain nominal calculi have been modeled with graph grammars, and hence their event structure semantics could be obtained as instances of the general case, the main limitation is that in the case of graph grammars the construction is more complex than strictly necessary for dealing with usual nominal calculi and, speaking in categorical terms, it is not as elegant as in the case of Petri nets. The main contribution of this work is the definition of a particular class of graph grammars, called persistent, that are expressive enough to model name passing calculi while simplifying the denotational domain construction, which can be expressed as an adjunction. Finally, we apply our technique to derive event structure semantics for pi-calculus and join-calculus processes. 1