Word problems requiring exponential time(Preliminary Report)

Abstract

The equivalence problem for Kleene's regular expressions has several effective solutions, all of which are computationally inefficient. In [1], we showed that this inefficiency is an inherent property of the problem by showing that the problem of membership in any arbitrary context-sensitive language was easily reducible to the equivalence problem for regular expressions. We also showed that with a squaring abbreviation ( writing (E)2 for E×E) the equivalence problem for expressions required computing space exponential in the size of the expressions. In this paper we consider a number of similar decidable word problems from automata theory and logic whose inherent computational complexity can be precisely characterized in terms of time or space requirements on deterministic or nondeterministic Turing machines. The definitions of the word problems and a table summarizing their complexity appears in the next section. More detailed comments and an outline of some of the proofs follows in the remaining sections. Complete proofs will appear in the forthcoming papers [9, 10, 13]. In the final section we describe some open problems.