Abstract
In this paper we consider time and space complexity of languages recognized by two-way deterministic pushdown automata with k input heads (2dpda(k)‘s, for short). Using the pyramidal structure of Schonhage [7] we design an algorithm simulating 2dpda(k)‘s in O(r) time and space, where r is the number of reachable (during the computation for a given input word) surface configurations. If the computation of 2dpda(k) is ‘sparse’, then r is an improvement upon O(nk). This generalizes the result of [4]. (As a by-product we give a new simple O(nk) time simulation of Zdpda(k)‘s.) We also indicate how to obtain a similar result for two-way nondeterministic pushdown automata. Our model of the computation is a random access machine with the uniform cost criterion.
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