Regular Expressions for PCTL Counterexamples

Counterexamples for probabilistic reachability in Markov chains are sets of paths that all reach a goal state and whose cumulative likelihood exceeds a threshold. This paper is concerned with the issue of how to conveniently represent these sets. Experiments, partially substantiated with combinatorial arguments, show that the cardinality of such sets may be excessive. To obtain compact representations of counterexamples we suggest to use regular expressions. We present a simple algorithm to generate minimal regular expressions and adopt a recursive scheme to determine their likelihood. Markov chain reduction prior to counter example generation may yield even shorter regular expressions. The feasibility of the approach is illustrated by means two protocols: leader election and the Crowds protocol.