We present an efficient algorithm for bisimulation equivalence. Generally, bisimulation equivalence can be tested in O(mn) for a labeled transition system with m transitions and n states. In order to come up with a more efficient algorithm, we establish a relationship between bisimulation equivalence and the relational coarsest partition problem, solved by Paige & Tarjan in O(m log n) time. Given an initial partition and a binary relation, the problem is to find the coarsest partition compatible with them. Computing bisimulation equivalence can be viewed both as an instance and as a generalization of this problem: an instance, because only the universal partition is considered as an initial partition and a generalization since we want to find a partition compatible with a family of binary relations instead of one single binary relation. We describe how we have adapted the Paige & Tarjan algorithm of complexity O(m log n) to minimize labeled transition systems modulo bisimulation equivalence. This algorithm has been implemented in C and is used in Aldebaran, a tool for the verification of concurrent systems.

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Reo is an exogenous coordination language for compositional construction of component connectors based on a calculus of channels. Building automated tools to address such concerns as equivalence or containment of the behavior of two given connectors, verification of the behavior of a connector, etc. requires an operational semantic model suitable for model checking. In this paper we introduce constraint automata and propose them as a semantic model for Reo.

Markovian Process Algebra (MPA) is a process algebra enhanced with exponential timing which allows the mapping of specifications on continuous time Markov chains (CTMCs). This paper introduces a compositional approach to compute the generator matrix of the CTMC underlying a MPA specification which consists of the parallel composition of finite state agents. Furthermore two different equivalence relations covering quantitative and qualitative aspects are introduced. These equivalence relations are shown to be congruences according to parallel composition of agents.

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Stochastic Automata Networks (SANs) are an efficient means to describe and analyze parallel systems under Markovian assumptions. The main advantage of SANs is the possibility to describe and analyze a complex parallel system in a compositional way such that the transition matrix of the Markov chain underlying the complete SAN can be described in a compositional way using only small matrices specifying single automata and combine these matrices by means of tensor operations. This approach allows, up to a certain extent, the handling of the state space explosion resulting from complex Markov models. In this paper equivalence relations for stochastic automata are introduced such that an automaton in a network can be substituted by an equivalent and usually smaller automaton without affecting the results of an analysis. We consider equivalence according to several performance quantities and for stationary and transient analysis of SANs. 1 Introduction Stochastic Automata Networks (SANs) u...

Formal methods are necessary in achieving correct software: that is, software that can be proven to fulfil its requirements. Formal specifications are unambiguous and analysable. Building a formal model improves understanding. The modelling of nondeterminism, and its subsequent removal in formal steps, allows design and implementation decisions to be made when most suitable. Formal models are amenable to mathematical manipulation and reasoning, and facilitate rigorous testing procedures. However, formal methods are not widely used in software development. In most cases, this is because they are not suitably supported with development tools. Further, many software developers do not recognise the need for rigour. Object oriented techniques are successful in the production of large, complex software systems. The methods are based on simple mathematical models of abstraction and classification. Further, the object oriented approach offers a conceptual consistency across all stages of soft...

This paper deals with probabilistic and nondeterministic processes represented by a variant of labelled transition systems where any outgoing transition of a state s is augmented with probabilities for the possible successor states. Our main contribution are algorithms for computing the bisimulation equivalence classes as introduced by Larsen & Skou [44] and the simulation preorder `a la Segala & Lynch [57]. The algorithm for deciding bisimilarity is based on a variant of the traditional partitioning technique [43, 51] and runs in time O(mn(log m+ log n)) where m is the number of transitions and n the number of states. The main idea for computing the simulation preorder is the reduction to maximum flow problems in suitable networks. Using the method of Cheriyan, Hagerup & Mehlhorn [15] for computing the maximum flow, the algorithm runs in time O((mn 6 +m 2 n 3 )= log n). Moreover, we show that the network-based technique is also applicable to compute the simulation-like relation...

. Various models and equivalence relations or preorders for probabilistic processes are proposed in the literature. This paper deals with a model based on labelled transition systems extended to the probabalistic setting and gives an O(n 2 \Delta m) algorithm for testing probabilistic bisimulation and an O(n 5 \Delta m 2 ) algorithm for testing probabilistic simulation where n is the number of states and m the number of transitions in the underlying probabilistic transition systems. 1 Introduction Transition systems have proved to be very useful for modelling concurrent processes. A variety of widely accepted equivalence relations and preorders for such systems support the use of transition systems for the design and verification of concurrent systems. In this context, testing equivalences and preorders become important and have been studied e.g. in [3, 4, 8, 11, 17]. For instance, (strong) bisimulation can be decided in time O(m \Delta log n) [22], weak bisimulation in t...

The analysis of programs by the exhaustive inspection of reachable states in a finite state graph is a well-understood procedure. It is straightforwardly applicable to many description languages and is actually implemented in several industrial tools. But one of the main limitations of today's verification tools is the size of the memory needed to exhaustively build the state graphs of the programs. For numerous properties, it is not necessary to explicitly build this graph and an exhaustive depth--first traversal is often sufficient. This leads to an on--line algorithms for computing Buchi acceptance (in the deterministic case) and behavioral equivalences: they are presented in detail. In order to avoid retraversing states, it is however important to store some of the already visited states in memory. To keep the memory size bounded (and avoid a performance falling down), visited states are randomly replaced. In most cases this depth--first traversal with replacement ca...