Deciding the inequivalence of context-free grammars with 1-letter terminal alphabet is S2p-complete

Abstract

This paper deals with the complexity of context-free grammars with 1-1etter terminal alphabet. We study the complexity of the membership problem and the inequivalence problem. We show that the first problem is NP-complete and the second one is Σ2p- complete with respect to log-space reduction. The second result also implies that the inequivalence problem is in PSPACE, solving an open problem stated in [3] by Hunt III, Rosenkrantz and Szymanski.