The complexity of the first-order theory of pure equality

Abstract

We will find a lower bound on the recognition complexity of the theories that are nontrivial relative to some equivalence relation (this relation may be equality), namely, each of these theories is consistent with the formula, whose sense is that there exist two non-equivalent elements. However, at first, we will obtain a lower bound on the computational complexity for the first-order theory of Boolean algebra that has only two elements. For this purpose, we will code the long-continued deterministic Turing machine computations by the relatively short-length quantified Boolean formulae; the modified Stockmeyer and Meyer method will appreciably be used for this simulation. Then, we will transform the modeling formulae of the theory of this Boolean algebra to the simulation ones of the first-order theory of the only equivalence relation in polynomial time. Since the computational complexity of these theories is not polynomial, we obtain that the class $\mathbf{P}$ is a proper subclass of $\mathbf{PSPACE}$ (Polynomial Time is a proper subset of Polynomial Space). Keywords: Computational complexity, the theory of equality, the coding of computations, simulation by means formulae, polynomial time, polynomial space, lower complexity bound