Strict Basic

In this paper we solve a long-standing open problem by showing that strict superposition|that is, superposition without equality factoring|is refutationally complete. The diiculty of the problem arises from the fact that the strict calculus, in contrast to the standard calculus with equality factoring, is not compatible with arbitrary removal of tautologies, so that the usual techniques for proving the (refutational) completeness of paramodulation calculi are not directly applicable. We deal with the problem by introducing a suitable notion of direct rewrite proof and modifying proof techniques based on candidate models and counterexamples in that we deene these concepts in terms of, not semantic truth, but direct provability. We introduce a corresponding concept of redundancy with which strict superposition is compatible and that covers most simpliication techniques. We also show that certain superposition inferences from variables are redundant|a result that is relevant, surprisingly, in the context of equality elimination methods.