Two paradigmatic algorithmic problems motivated by formal verification that are in the focus of this thesis are model checking and bisimilarity checking. In the thesis game theoretic formulations of the problems are used to abstract away from syntactic and semantic peculiarities of formal models and specification formalisms studied. This facilitates a detailed algorithmic analysis, leading to two novel model checking algorithms with better theoretical or practical performance, and to an undecidability result for a notion of bisimilarity.
The original technical contributions of this thesis are collected in three research articles whose revised and extended versions are included in the dissertation.
In the first two papers the computational complexity of deciding the winner in parity games is studied. The problem of solving parity games is polynomial time equivalent to the modal mu-calculus model checking. The modal mu-calculus plays a central role in the study of logics for specification and verification of programs. The model checking problem is extensively studied in literature on computer aided verification. The question whether there is a polynomial time algorithm for the modal mu-calculus model checking is one of the most challenging and fascinating open questions in the area.
In the first paper a new algorithm is developed for solving parity games, and hence for the modal mu-calculus model checking. The design and analysis of the algorithm are based on a semantic notion of a progress measure. The worst-case running time of the resulting algorithm matches the best worst-case running time bounds known so far for the problem, achieved by the algorithms due to Browne at al., and Seidl. Our algorithm has better space complexity: it works in small polynomial space; the other two algorithms have exponential worst-case space complexity.
In the second paper a novel approach to model checking is pursued, based on a link between parity games and discounted payoff and stochastic games, established and advocated by Puri. A discrete strategy improvement algorithm is given for solving parity games, thereby proving a new procedure for the modal mu-calculus model checking. Known strategy improvement algorithms, as proposed for stochastic games by Hoffman and Karp, and for discounted payoff games and parity games by Puri, work with real numbers and require solving linear programming instances involving high precision arithmetic. The present algorithm for parity games avoids these difficulties by efficient manipulation of carefully designed discrete valuations. A fast implementation is given for a strategy improvement step. Another advantage of the approach is that it provides a better conceptual understanding of the underlying discrete structure and gives hope for easier analysis of strategy improvement algorithms for parity games. However, so far it is not known whether the algorithm works in polynomial time. The long standing problem whether parity games can be solved in polynomial time remains open.
In the study of concurrent systems it is common to model concurrency by non-determinism. There are, however, some models of computation in which concurrency is represented explicitly; elementary net systems and asynchronous transition systems are well-known examples. History preserving and hereditary history preserving bisimilarities are behavioural equivalence notions taking into account causal relationships between events of concurrent systems. Checking history preserving bisimilarity is known to be decidable for finite labelled elementary nets systems and asynchronous transition systems. Its hereditary version appears to be only a slight strengthening and it was conjectured to be decidable too. In the third paper it is proved that checking hereditary history preserving bisimilarity is undecidable for finite labelled asynchronous transition systems and elementary net systems. This solves a problem open for several years. The proof is done in two steps. First an intermediate problem of deciding the winner in domino bisimulation games for origin constrained tiling systems is introduced and its undecidability is shown by a reduction from the halting problem for 2-counter machines. Then undecidability of hereditary history preserving bisimilarity is shown by a reduction from the problem of domino bisimulation games.
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