We consider automata systems consisting of several pushdown automata working in parallel and communicating the contents of their stacks by request, using a communication strategy borrowed from grammar system theory. We investigate the computational power of these mechanisms. We prove that non-centralized parallel communicating pushdown automata systems with a bounded number of components, where each automaton is allowed to issue a query, are able to recognize all recursively enumerable languages. We also present homomorphical characterizations of the class of recursively enumerable languages for the centralized variants, where only a distinguished automaton issues queries. Moreover, we show that these centralized variants are at least as powerful as one-way multihead pushdown automata. Finally, some open problems and further directions of research are discussed.Keywords: parallel communicating systems, pushdown automata, two-stack pushdown automata, n-head pushdown automata 633 Int. J. Found. Comput. Sci. 2000.11:631-650. Downloaded from www.worldscientific.com by UNIVERSITY OF QUEENSLAND on 08/15/15. For personal use only.
We study two very simple variants of P colonies: systems with only one object inside the cells, and systems with insertion-deletion programs, so called P colonies with senders and consumers. We show that both of these extremely simple types of systems are able to compute any recursively enumerable set of vectors of non-negative integers.
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