Locality Testing for NFAs is PSPACE-complete

Abstract: The class of local languages is a well-known subclass of the regular languages that admits many equivalent characterizations. In this short note we establish the PSPACE-completeness of the problem of determining, given as input a nondeterministic finite automaton (NFA) A, whether the language recognized by A is local or not. This contrasts with the case of deterministic finite automata (DFA), for which the problem is known to be in PTIME.