Sequential Composition in the Presence of Intermediate Termination (Extended Abstract)

The standard operational semantics of the sequential composition operator gives rise to unbounded branching and forgetfulness when transparent process expressions are put in sequence. Due to transparency, the correspondence between context-free and pushdown processes fails modulo bisimilarity, and it is not clear how to specify an always terminating half counter. We propose a revised operational semantics for the sequential composition operator in the context of intermediate termination. With the revised operational semantics, we eliminate transparency, allowing us to establish a close correspondence between context-free processes and pushdown processes. Moreover, we prove the reactive Turing powerfulness of TCP with iteration and nesting with the revised operational semantics for sequential composition.Comment: In Proceedings EXPRESS/SOS 2017, arXiv:1709.00049. arXiv admin note: substantial text overlap with arXiv:1706.0840

Parallel Pushdown Automata and Commutative Context-Free Grammars in Bisimulation Semantics (Extended Abstract)

A classical theorem states that the set of languages given by a pushdown automaton coincides with the set of languages given by a context-free grammar. In previous work, we proved the pendant of this theorem in a setting with interaction: the set of processes given by a pushdown automaton coincides with the set of processes given by a finite guarded recursive specification over a process algebra with actions, choice, sequencing and guarded recursion, if and only if we add sequential value passing. In this paper, we look what happens if we consider parallel pushdown automata instead of pushdown automata, and a process algebra with parallelism instead of sequencing.Comment: In Proceedings EXPRESS/SOS2023, arXiv:2309.05788. arXiv admin note: text overlap with arXiv:2203.0171

Parallel Pushdown Automata and Commutative Context-Free Grammars in Bisimulation Semantics

A classical theorem states that the set of languages given by a pushdown automaton coincides with the set of languages given by a context-free grammar. In previous work, we proved the pendant of this theorem in a setting with interaction: the set of processes given by a pushdown automaton coincides with the set of processes given by a finite guarded recursive specification over a process algebra with actions, choice, sequencing and guarded recursion, if and only if we add sequential value passing. In this paper, we look what happens if we consider parallel pushdown automata instead of pushdown automata, and a process algebra with parallelism instead of sequencing.</p

Sequential Composition in the Presence of Intermediate Termination (Extended Abstract)

The standard operational semantics of the sequential composition operator gives rise to unbounded branching and forgetfulness when transparent process expressions are put in sequence. Due to transparency, the correspondence between context-free and pushdown processes fails modulo bisimilarity, and it is not clear how to specify an always terminating half counter. We propose a revised operational semantics for the sequential composition operator in the context of intermediate termination. With the revised operational semantics, we eliminate transparency, allowing us to establish a close correspondence between context-free processes and pushdown processes. Moreover,we prove the reactive Turing powerfulness of TCP with iteration and nesting with the revised operational semantics for sequential composition

Interviews with the 2021 CONCUR Test-of-Time Award recipients

Last year, the CONCUR conference series inaugurated its Test-of-Time Award, purpose of which is to recognise important achievements in Con- currency Theory that were published at the CONCUR conference and that have stood the test of time. This year, the following four papers were chosen to receive the CONCUR Test-of-Time Awards for the periods 1994–1997 and 1996–1999 by a jury consisting of Rob van Glabbeek (chair), Luca de Alfaro, Nathalie Bertrand, Catuscia Palamidessi, and Nobuko Yoshida: - David Janin and Igor Walukiewicz. On the Expressive Completeness of the Propositional mu-Calculus with respect to Monadic Second Or- der Logic [3]. - Uwe Nestmann and Benjamin C. Pierce. Decoding Choice Encod- ings [4]. - Ahmed Bouajjani, Javier Esparza, and the late Oded Maler. Reacha- bility Analysis of Pushdown Automata: Application to Model- checking [2]. - Rajeev Alur, Thomas A. Henzinger, Orna Kupferman, and Moshe Y. Vardi. Alternating Refinement Relations [1]. This year, the second paper was live-interviewed by Nobuko Yoshida; the third paper was interviewed by Nathalie Bertrand and the forth paper was interviewed by Luca Aceto. Adam Barwell and Francisco Ferreira helped making the article from the live interview by Yoshida

Parallel pushdown automata and commutative context-free grammars in bisimulation semantics

A classical theorem states that the set of languages given by a pushdown automaton coincides with the set of languages given by a context-free grammar. In previous work, we proved the pendant of this theorem in a setting with interaction: the set of processes given by a pushdown automaton coincides with the set of processes given by a finite guarded recursive specification over a process algebra with actions, choice, sequencing and guarded recursion, if and only if we add sequential value passing. In this paper, we look what happens if we consider parallel pushdown automata instead of pushdown automata, and a process algebra with parallelism instead of sequencing

Better abstractions for timed automata

We consider the reachability problem for timed automata. A standard solution to this problem involves computing a search tree whose nodes are abstractions of zones. These abstractions preserve underlying simulation relations on the state space of the automaton. For both effectiveness and efficiency reasons, they are parametrized by the maximal lower and upper bounds (LU-bounds) occurring in the guards of the automaton. We consider the aLU abstraction defined by Behrmann et al. Since this abstraction can potentially yield non-convex sets, it has not been used in implementations. We prove that aLU abstraction is the biggest abstraction with respect to LU-bounds that is sound and complete for reachability. We also provide an efficient technique to use the aLU abstraction to solve the reachability problem.Comment: Extended version of LICS 2012 paper (conference paper till v6). in Information and Computation, available online 27 July 201

Towards a Generic Model Theory: Automatic Bisimulations for Atomic, Molecular and First-order Logics

After observing that the truth conditions of connectives of non-classical logics are generally defined in terms of formulas of first-order logic (FOL), we introduce protologics, a class of logics whose connectives are defined by arbitrary first-order formulas. Then, we identify two subclasses of protologics which are particularly well-behaved. We call them atomic and molecular logics. Notions of invariance for atomic and molecular logics can be automatically defined from the truth conditions of their connectives, bisimulations do not need to be defined by hand on a case by case basis for each logic. Moreover, molecular logics behave as 'paradigmatic logics': every first-order logic and every protologic is as expressive as a molecular logic. Then, we prove a series of model-theoretical results for molecular logics which characterize them as fragments of FOL and which provide criteria for axiomatizability and definability of a class of models in these logics. In particular, we rediscover van Benthem's theorem for modal logic as a specific instance of our generic theorems and other results for modal intuitionistic logic and temporal logic. We also discover a wide range of novel results, such as for the Lambek calculus. Then, we apply our method and generic results to FOL and find out novel invariance notions for FOL, that we call predicate bisimulation and first-order bisimulation. They refine the usual notions of isomorphism and partial isomorphism. We prove generalizations as well as new versions of the Keisler theorems for countable languages in which isomorphisms are replaced by predicate bisimulations and first-order bisimulations