ON THE POWER OF BOUNDED CONCURRENCY .2. PUSHDOWN-AUTOMATA

This is the second in a series of papers on the inherent power of bounded cooperative concurrency, whereby an automaton can be in some bounded number of states that cooperate in accepting the input. In this paper, we consider pushdown automata. We are interested in differences in power of expression and in exponential (or higher) discrepancies in succinctness between variants of pda's that incorporate nondeterminism (E), pure parallelism (A), and bounded cooperative concurrency (C). Technically, the results are proved for cooperating pushdown automata with cooperating states, but they hold for appropriate versions of most concurrent models of computation. We exhibit exhaustive sets of upper and lower bounds on the relative succinctness of these features for three classes of languages: deterministic context-free, regular, and finite. For example, we show that C represents exponential savings in succinctness in all cases except when both E and A are present (i.e., except for alternating automata), and that E and A represent unlimited savings in succinctness in all cases.