A second-order system for polytime reasoning using Gradel's theorem

We introduce a second-order system V/sub 1/-Horn of bounded arithmetic formalizing polynomial-time reasoning, based on Gradel's (1992) second-order Horn characterization of P. Our system has comprehension over P predicates (defined by Gradel's second-order Horn formulas), and only finitely, many function symbols. Other systems of polynomial-time reasoning either allow induction on NP predicates (such as Buss's (1986) S/sub 2//sup 1/ or the second-order V/sub 1//sup 1/), and hence are more powerful than our system (assuming the polynomial hierarchy does not collapse), or use Cobham's theorem to introduce function symbols for all polynomial-time functions (such as Cook's PV and Zambella's P-def). We prove that our system is equivalent to QPV and Zambella's (1996) P-def. Using our techniques, we also show that V/sub 1/-Horn is finitely, axiomatizable, and, as a corollary, that the class of /spl forall//spl Sigma//sub 1//sup b/ consequences of S/sub 2//sup 1/ is finitely axiomatizable as well, thus answering an open question.