The Turing machine models an old-fashioned computer, that does not interact with the user or with other computers, and only does batch processing. Therefore, we came up with a Reactive Turing Machine that does not have these shortcomings. In the Reactive Turing Machine, transitions have labels to give a notion of interactivity. In the resulting process graph, we use bisimilarity instead of language equivalence. Subsequently, we considered other classical theorems and notions from automata theory and formal languages theory. In this paper, we consider the classical theorem of the correspondence between pushdown automata and context-free grammars. By changing the process operator of sequential composition to a sequencing operator with intermediate acceptance, we get a better correspondence in our setting. We find that the missing ingredient to recover the full correspondence is the addition of a notion of state awareness.

The Turing machine models an old-fashioned computer, that does not interact with the user or with other computers, and only does batch processing. Therefore, we came up with a Reactive Turing Machine that does not have these shortcomings. In the Reactive Turing Machine, transitions have labels to give a notion of interactivity. In the resulting process graph, we use bisimilarity instead of language equivalence. Subsequently, we considered other classical theorems and notions from automata theory and formal languages theory. In this paper, we consider the classical theorem of the correspondence between pushdown automata and context-free grammars. By changing the process operator of sequential composition to a sequencing operator with intermediate acceptance, we get a better correspondence in our setting. We find that the missing ingredient to recover the full correspondence is the addition of a notion of state awareness.

This short article announces the recipients of the CONCUR Test-of-Time Award 2020.

The Theory of Sequential Processes includes deadlock, successful termination, action prefixing, alternative and sequential composition. Intermediate acceptance, which is important for the integration of classical automata theory, can be expressed through a combination of alternative composition and successful termination. Recently, it was argued that complications arising from the interplay between intermediate acceptance and sequential composition can be eliminated by replacing sequential composition by sequencing. In this paper we study the equational theory of the recursion-free fragment of the resulting process theory modulo bisimilarity, proving that it is not finitely based, but does afford a ground-complete axiomatisation if a unary auxiliary operator is added. Furthermore, we prove that bisimilarity is decidable for processes definable by means of a finite guarded recursive specification over the process theory.

The languages accepted by finite automata are precisely the languages denoted by regular expressions. In contrast, finite automata may exhibit behaviours that cannot be described by regular expressions up to bisimilarity. In this paper, we consider extensions of the theory of regular expressions with various forms of parallel composition and study the effect on expressiveness. First we prove that adding pure interleaving to the theory of regular expressions strictly increases its expressiveness modulo bisimilarity. Then, we prove that replacing the operation for pure interleaving by ACP-style parallel composition gives a further increase in expressiveness, still insufficient, however, to facilitate the expression of all finite automata up to bisimilarity. Finally, we prove that the theory of regular expressions with ACP-style parallel composition and encapsulation is expressive enough to express all finite automata up to bisimilarity. Our results extend the expressiveness results obtained by Bergstra, Bethke and Ponse for process algebras with (the binary variant of) Kleene's star operation.

Abstract We conservatively extend an ACP-style discrete-time process theory with discrete stochastic delays. The semantics of the timed delays relies on time additivity and time determinism, which are properties that enable us to merge subsequent timed delays and to impose their synchronous expiration. Stochastic delays, however, interact with respect to a so-called race condition that determines the set of delays that expire first, which is guided by an (implicit) probabilistic choice. The race condition precludes the property of time additivity as the merger of stochastic delays alters this probabilistic behavior. To this end, we resolve the race condition using conditionally-distributed unit delays. We give a sound and ground-complete axiomatization of the process theory comprising the standard set of ACP-style operators. In this generalized setting, the alternative composition is no longer associative, so we have to resort to special normal forms that explicitly resolve the underlying race condition. Our treatment succeeds in the initial challenge to conservatively extend standard time with stochastic time. However, the ‘dissection’ of the stochastic delays to conditionally-distributed unit delays comes at a price, as we can no longer relate the resolved race condition to the original stochastic delays. We seek a solution in the field of probabilistic refinements that enable the interchange of probabilistic and nondeterministic choices.

The three classical process algebras CCS, CSP and ACP present several differences in their respective technical machinery. This is due, not only to the difference in their operators, but also to the terminology and ‘way of thinking’ of the community that has been (and still is) working with them. In this paper we will first discuss these differences and try to clarify the different usage of terminology and concepts. Then, as a result of this discussion, we define a generic process algebra where each of the basic mechanisms of the three process algebras (including minimal fixpoint based unguarded recursion) is expressed by an operator, and which can be used as an underlying common language. We show an example of the advantages of adopting such a language instead of one of the three more specialised algebras: producing a complete axiomatisation for Milner's observational congruence in the presence of (unguarded) recursion and static operators. More precisely, we provide a syntactical characterisation (allowing as many terms as possible) for the equations involved in recursion operators, which guarantees that transition systems generated by the operational semantics are finite state. Conversely, we show that every process admits a specification in terms of such a restricted form of recursion. We then present an axiomatisation that is ground complete over such a restricted signature. Notably, we also show that the two standard axioms of Milner for weakly unguarded recursion can be expressed using a single axiom only.

We solve an open question of Milner [1984]. We define a set of so-called well-behaved finite automata that, modulo bisimulation equivalence, corresponds exactly to the set of regular expressions, and we show how to determine whether a given finite automaton is in this set. As an application, we consider the star height problem.

We tackle an open question of Milner (1984). We define a set of so-called well-behaved finite automata that, modulo bisimulation equivalence, corresponds exactly to the set of regular expressions.

In ACP-style process algebra the interpretation of a constant atomic action combines action execution with termination. In a setting with timing, different forms of termination can be distinguished: some-time termination, termination before the next clock tick, urgent termination, having terminated. In a setting with the silent action for successful termination (skip). We can recover standard ACP-style process algebras as subtheories of the new theory. The new approach has definite advantages over the standard approach. The paper contributes to ongoing work on relationships between algebras with different timing features.

We discuss the key notions of discrete time process algebra in the setting of ACP. Time is measured in discrete slices. The emphasis is on absolute, relative and parametric time notation. Note: Partial support received from ESPRIT Basic Research Action 7166, C0NCUR2.

We consider processes that have transitions labeled with atomic actions, and states labeled with formulas over a propositional logic. These state labels are called signals. A process in a parallel composition may proceed conditionally, dependent on the presence of a signal in the process in parallel. This allows a natural treatment of signal observation

Abstract The axiom system ACP of [BeK84a] was extended with real time features in [BaB91]. Here we proceed to define a discrete time extension of ACP, along the lines of ATP [NiS94]. We present versions based on relative timing and on absolute timing. Both approaches are integrated using parametric timing. The time free ACP theory is embedded in the discrete time theory.

Abstract We illustrate the difference between sequential composition in process algebra axiomatisations like ACP and action prefixing in process calculi like CCS. We define both early and late input in a general framework extending ACP, and consider various subalgebras, some very close to value passing CCS, another one close to CSP.

Abstract The real time process algebra of Baeten and Bergstra [ Formal Aspects of Computing , 3 , 142–188 (1991)] is extended to real space by requiring the presence of spatial coordinates for each atomic action, in addition to the required temporal attribute. It is found that asynchronous communication cannot easily be avoided. Based on the state operators of Baeten and Bergstra [ Information and Computation , 78 , 205–245 (1988)] and following Bergstra et al. [ Proc. Seminar on Concurrency , LNCS 197, Springer, 1985, pp. 76–95], asychronous communication mechanisms are introduced as an additional feature of real space process algebra. The overall emphasis is on the introductory explanation of the features of real space process algebra, and characteristic examples are given for each of these.

article Free AccessDecidability of bisimulation equivalence for process generating context-free languages Authors: J. C. M. Baeten Univ. of Amsterdam, Amsterdam, The Netherlands Univ. of Amsterdam, Amsterdam, The NetherlandsView Profile , J. A. Bergstra Univ. of Amsterdam, Amsterdam, The Netherlands; and State Univ. of Utrecht, Utrecht, The Netherlands Univ. of Amsterdam, Amsterdam, The Netherlands; and State Univ. of Utrecht, Utrecht, The NetherlandsView Profile , J. W. Klop CWI, Amsterdam, The Netherlands; and Free Univ., Amsterdam, The Netherlands CWI, Amsterdam, The Netherlands; and Free Univ., Amsterdam, The NetherlandsView Profile Authors Info & Claims Journal of the ACMVolume 40Issue 3July 1993 pp 653–682https://doi.org/10.1145/174130.174141Published:01 July 1993Publication History 96citation724DownloadsMetricsTotal Citations96Total Downloads724Last 12 Months22Last 6 weeks7 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

Abstract We describe an axiom system ACP p that incorporates real timed actions. Many examples are provided in order to explain the intuitive contents of the notation. ACP p is a generalisation of ACP. This implies that some of the axioms have to be relaxed and that ACP can be recovered as a special case from it. The purpose of ACP p is to serve as a specification language for real time systems. The axioms of ACP p explain its operational meaning in an algebraic form.

We consider a process semantics intermediate between bi-simulation semantics and readiness semantics, called here ready-trace semantics. The advantage of this semantics is that, while retaining the simplicity of readiness semantics, it is still possible to augment this process model with the mechanism of atomic actions with priority (the θ operator). It is shown that in readiness semantics and a fortiori in failure semantics such an extension with θ is impossible. Ready-trace semantics is considered here in the simple setting of concrete process algebra, that is: without abstraction (no silent moves), moreover for finite processes only. For such finite processes without silent moves a complete axiomatisation of ready-trace semantics is given via the method of process graph transformations.