The undecidability of proof search when equality is a logical connective

Abstract

One proof-theoretic approach to equality in quantificational logic treats equality as a logical connective: in particular, term equality can be given both left and right introduction rules in a sequent calculus proof system. We present a particular example of this approach to equality in a first-order logic setting in which there are no predicate symbols (apart from equality). After we illustrate some interesting applications of this logic, we show that provability in this logic is undecidable.