Asynchronous Parallel Communicating Systems of Pushdown Automata

Abstract

We introduce asynchronous variants of the parallel communicating systems of pushdown automata of Csuhaj-Varjú et al. These are obtained by using a response symbol in addition to the usual query symbols. Our main result states that centralized asynchronous parallel communicating systems of pushdown automata of degree n that work in returning mode have exactly the same expressive power as n-head pushdown automata. This holds in the nondeterministic as well as in the deterministic case. In addition, it is shown that the class of binary relations that is computed by centralized asynchronous parallel communicating systems of pushdown automata of degree two that are working in returning mode coincides with the class of pushdown relations.