The Complexity of Positive First-Order Logic without Equality

Abstract

We study the complexity of evaluating positive equality-free sentences of first-order (FO) logic over a fixed, finite structure B . This may be seen as a natural generalisation of the nonuniform quantified constraint satisfaction problem QCSP( B ). We introduce surjective hyper-endomorphisms and use them in proving a Galois connection that characterizes definability in positive equality-free FO. Through an algebraic method, we derive a complete complexity classification for our problems as B ranges over structures of size at most three. Specifically, each problem either is in L, is NP-complete, is co-NP-complete, or is Pspace-complete.