Pushdown Specifications

Traditionally, model checking is applied to finite-state systems and regular specifications. While researchers have successfully extended the applicability of model checking to infinite-state systems, almost all existing work still consider regular specification formalisms. There are, however, many interesting non-regular properties one would like to model check. In this paper we study model checking of pushdown specifications. Our specification formalism is nondeterministic pushdown parity tree automata (PD-NPT). We show that the model-checking problem for regular systems and PD-NPT specifications can be solved in time exponential in the system and the specification. Our model-checking algorithm involves a new solution to the nonemptiness problem of nondeterministic pushdown tree automata, where we improve the best known upper bound from a tripleexponential to a single exponential. We also consider the model-checking problem for context-free systems and PD-NPT specifications and show that it is undecidable. Address: School of Computer Science and Engineering, Hebrew University, Jerusalem 91904, Israel. Email: [email protected] Department of Computer Science and Applied Mathematics, Weizmann institute, Rehovot 76100, Israel. Email: [email protected] Address: Department of Computer Science, Rice University, Houston TX 77005-1892, U.S.A. Email: [email protected]