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Bisimilarity of Open Terms
, 2000
"... Traditionally, in process calculi, relations over open terms, i.e., terms with free process variables, are defined as extensions of closedterm relations: two open terms are related if and only if all their closed instantiations are related. Working in the context of bisimulation, in this paper we s ..."
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Traditionally, in process calculi, relations over open terms, i.e., terms with free process variables, are defined as extensions of closedterm relations: two open terms are related if and only if all their closed instantiations are related. Working in the context of bisimulation, in this paper we study a different approach; we define semantic models for open terms, socalled conditional transition systems, and define bisimulation directly on those models. It turns out that this can be done in at least two different ways, one giving rise to De Simone's formal hypothesis bisimilarity and the other to a variation which we call hypothesispreserving bisimilarity (denoted t fh and t hp, respectively). For open terms, we have (strict) inclusions t fh /t hp / t ci (the latter denoting the standard ``closed instance' ' extension); for closed terms, the three coincide. Each of these relations is a congruence in the usual sense. We also give an alternative characterisation of t hp in terms of nonconditional transitions, as substitutionclosed bisimilarity (denoted t sb). Finally, we study the issue of recursion congruence: we prove that each of the above relations is a congruence with respect to the recursion operator; however, for t ci this result holds under more restrictive conditions than for tfh and thp.]
Some of My Favourite Results in Classic Process Algebra
 In Bulletin of the EATCS
, 2003
"... this paper has generated a veritable industry of results on the metatheory of SOS and process algebras. (See [10] for a mention of some of these achievements and pointers to the original literature.) The proof techniques used in these results were extremely ingenious, and have paved the way to many ..."
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Cited by 9 (3 self)
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this paper has generated a veritable industry of results on the metatheory of SOS and process algebras. (See [10] for a mention of some of these achievements and pointers to the original literature.) The proof techniques used in these results were extremely ingenious, and have paved the way to many similar developments. Again, the role played by the modal characterizations of behavioural equivalences in the proof of the characterizations of the largest congruences is remarkable
Axiomatizations for the perpetual loop in process algebra
 Proc. 24 th ICALP, pages 571581. Volume 1256 of Lecture
, 1997
"... Abstract. Milner proposed an axiomatization for the Kleene star in basic process algebra, in the presence of deadlock and empty process, modulo bisimulation equivalence. In this paper, Milner’s axioms are adapted to noexit iteration x ω, which executes x infinitely many times in a row, and it is sh ..."
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Abstract. Milner proposed an axiomatization for the Kleene star in basic process algebra, in the presence of deadlock and empty process, modulo bisimulation equivalence. In this paper, Milner’s axioms are adapted to noexit iteration x ω, which executes x infinitely many times in a row, and it is shown that this axiomatization is complete for noexit iteration in basic process algebra with deadlock and empty process, modulo bisimulation. 1
Ordered SOS Process Languages for Branching and Eager Bisimulations
 INFORMATION AND COMPUTATION
, 2002
"... We present a general and uniform method for defining structural operational semantics (SOS) of process operators by traditional Plotkinstyle transition rules equipped with orderings. This new feature allows one to control the order of application of rules when deriving transitions of process terms. ..."
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We present a general and uniform method for defining structural operational semantics (SOS) of process operators by traditional Plotkinstyle transition rules equipped with orderings. This new feature allows one to control the order of application of rules when deriving transitions of process terms. Our method is powerful enough to deal with rules with negative premises and copying. We show that rules with orderings, called ordered SOS rules, have the same expressive power as GSOS rules. We identify several classes of process languages with operators defined by rules with and without orderings in the setting with silent actions and divergence. We prove that branching bisimulation and eager bisimulation relations are preserved by all operators in process languages in the relevant classes.
An Axiomatization for the Terminal Cycle
 Utrecht University
, 1996
"... Milner proposed an axiomatization for the Kleene star in basic process algebra, in the presence of deadlock and empty process, modulo bisimulation equivalence. In this paper, Milner's axioms are adapted to the terminal cycle x ! , which executes x infinitely many times in a row, and it is shown t ..."
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Milner proposed an axiomatization for the Kleene star in basic process algebra, in the presence of deadlock and empty process, modulo bisimulation equivalence. In this paper, Milner's axioms are adapted to the terminal cycle x ! , which executes x infinitely many times in a row, and it is shown that this axiomatization is complete for the terminal cycle in basic process algebra with deadlock and empty process modulo bisimulation. 1 Introduction Kleene [15] defined a binary operator x y in the context of finite automata, which denotes the iterate of x and y. Intuitively, the expression x y can choose to execute either x, after which it evolves into x y again, or y, after which it terminates. A feature of the Kleene star is that on the one hand it can express recursion, while on the other hand it can be captured in equational laws. Hence, one does not need metaprinciples such as the Recursive Specification Principle from Bergstra and Klop [10]. Kleene formulated several equa...
Is Observational Congruence on µExpressions Axiomatisable in Equational Horn Logic?
, 2007
"... It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom/Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either t ..."
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It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom/Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either the rules are impure by involving nonequational sideconditions, or they are schematically infinitary like the congruence rule which is not Horn. It is an open question whether these complications cannot be avoided in the prooftheoretically and computationally clean and powerful setting of secondorder equational Horn logic. This paper presents a positive and a negative result regarding axiomatisability of observational congruence in equational Horn logic. Firstly, we show how Milner’s impure rule system can be reworked into a pure Horn axiomatisation that is complete for guarded processes. Secondly, we prove that for unguarded processes, both Milner’s and Bloom/Ésik’s axiomatisations are incomplete without the congruence rule, and neither system has a complete extension in rank 1 equational axioms. It remains open whether there are higherrank equational axioms or Horn rules which would render Milner’s or Bloom / Ésik’s axiomatisations complete for unguarded processes.