Quantified Propositional Logspace Reasoning

Abstract

In this paper, we develop a quantified propositional proof systems that corresponds to logarithmic-space reasoning. We begin by defining a class SigmaCNF(2) of quantified formulas that can be evaluated in log space. Then our new proof system GL^* is defined as G_1^* with cuts restricted to SigmaCNF(2) formulas and no cut formula that is not quantifier free contains a free variable that does not appear in the final formula. To show that GL^* is strong enough to capture log space reasoning, we translate theorems of VL into a family of tautologies that have polynomial-size GL^* proofs. VL is a theory of bounded arithmetic that is known to correspond to logarithmic-space reasoning. To do the translation, we find an appropriate axiomatization of VL, and put VL proofs into a new normal form. To show that GL^* is not too strong, we prove the soundness of GL^* in such a way that it can be formalized in VL. This is done by giving a logarithmic-space algorithm that witnesses GL^* proofs.