Sp Bas Luttik

Towards Model Checking Executable UML Specifications in mCRL2

We describe a translation of a subset of executable UML (xUML) into the process algebraic specification language mCRL2. This subset includes class diagrams with class generalisations, and state machines with signal and change events. The choice of these xUML constructs is dictated by their use in the modelling of railway interlocking systems. The long-term goal is to verify safety properties of interlockings modelled in xUML using the mCRL2 and LTSmin toolsets. Initial verification of an interlocking toy example demonstrates that the safety properties of model instances depend crucially on the run-to-completion assumptions.

A finite equational base for CCS with left merge and communication merge

Using the left merge and the communication merge from ACP, we present an equational base (i.e., a ground-complete and ω-complete set of valid equations) for the fragment of CCS without recursion, restriction and relabeling modulo (strong) bisimilarity. Our equational base is finite if the set of actions is finite.

CCS with Hennessy's merge has no finite-equational axiomatization

This paper confirms a conjecture of Bergstra and Klops from 1984 by establishing that the process algebra obtained by adding an auxiliary operator proposed by Hennessy in 1981 to the recursion free fragment of Milners Calculus of Communicating Systems is not finitely based modulo bisimulation equivalence. Thus, Hennessys merge cannot replace the left merge and communication merge operators proposed by Bergstra and Klop, at least if a finite axiomatization of parallel composition modulo bisimulation equivalence is desired.

Reactive turing machines

We propose reactive Turing machines (RTMs), extending classical Turing machines with a process-theoretical notion of interaction. We show that every effective transition system is simulated modulo branching bisimilarity by an RTM, and that every computable transition system with a bounded branching degree is simulated modulo divergencepreserving branching bisimilarity. We conclude from these results that the parallel composition of (communicating) RTMs can be simulated by a single RTM. We prove that there exist universal RTMs modulo branching bisimilarity, but these essentially employ divergence to be able to simulate an RTM of arbitrary branching degree. We also prove that modulo divergence-preserving branching bisimilarity there are RTMs that are universal up to their own branching degree. Finally, we establish a correspondence between RTMs and the process theory TCPτ.

Finite equational bases for fragments of CCS with restriction and relabelling

We investigate the equational theory of several fragments of CCS modulo (strong) bisimilarity with special attention to restriction and relabelling. The largest fragment we consider includes action prefixing, choice, parallel composition without communication, restriction and relabelling. We present a finite equational base (i.e., a finite ground-complete and omega-complete axiomatisation) for it, including the left merge from ACP as auxiliary operation to facilitate the axiomatisation of parallel composition.

Unique parallel decomposition in branching and weak bisimulation semantics

We consider the property of unique parallel decomposition modulo branching and weak bisimilarity. First, we show that normed behaviours always have parallel decompositions, but that these are not necessarily unique. Then, we establish that finite behaviours have unique parallel decompositions. We derive the latter result from a general theorem about unique decompositions in partial commutative monoids.

Expressiveness modulo Bisimilarity of Regular Expressions with Parallel Composition

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.

A process-theoretic look at automata

Automata theory presents roughly three types of automata: finite automata, pushdown automata and Turing machines. The automata are treated as language acceptors, and the expressiveness of the automata models are considered modulo language equivalence. This notion of equivalence is arguably too coarse to satisfactorily deal with a notion of interaction that is fundamental to contemporary computing. In this paper we therefore reconsider the automaton models from automata theory modulo branching bisimilarity, a well-known behavioral equivalence from process theory that has proved to be able to satisfactorily deal with interaction. We investigate to what extent some standard results from automata theory are still valid if branching bisimilarity is adopted as the preferred equivalence.

Computations and interaction

We enhance the notion of a computation of the classical theory of computing with the notion of interaction. In this way, we enhance a Turing machine as a model of computation to a Reactive Turing Machine that is an abstract model of a computer as it is used nowadays, always interacting with the user and the world.

Unguardedness mostly means many solutions

Abstract A widely accepted method to specify (possibly infinite) behaviour is to define it as the solution, in some process algebra, of a recursive specification, i.e., a system of recursive equations over the fundamental operations of the process algebra. The method only works if the recursive specification has a unique solution in the process algebra; it is well-known that guardedness is a sufficient requirement on a recursive specification to guarantee a unique solution in any of the standard process algebras. In this paper we investigate to what extent guardedness is also a necessary requirement to ensure unique solutions. We prove a theorem to the effect that all unguarded recursive specifications over BPA have infinitely many solutions in the standard models for BPA . In contrast, we observe that there exist recursive specifications over PA , necessarily involving parallel composition, that have a unique solution, or finitely many solutions in the standard models for PA .