Monadic second-order unification Is NP-complete

Abstract

Monadic Second-Order Unification (MSOU) is Second-Order Unification where all function constants occurring in the equations are unary. Here we prove that the problem of deciding whether a set of monadic equations has a unifier is NP-complete. We also prove that Monadic Second-Order Matching is also NP-complete. © Springer-Verlag 2004.This research has been partially supported by the CICYT Research Projects CADVIAL (TIC2001-2392-C03-01) and LOGFAC (TIC2001-1577-C03-01)Peer Reviewe