We study possible formulations of algebraic propositional proofs operating with noncommutative polynomials written as algebraic noncommutative formulas. First, we observe that a simple formulation of such proof systems gives rise to systems at least as strong as Frege— yielding also a semantic way to define a Cook-Reckhow (i.e., polyno-mially verifiable) algebraic variant of Frege proofs, different from that given before in [8,11]. We then turn to an apparently weaker system, namely, Polynomial Calculus (PC) where polynomials are written as ordered formulas (PC over ordered formulas, for short). This is an algebraic propositional proof system that operates with noncommutative polynomials in which the order of products in all monomials respects a fixed linear ordering on the variables, and where proof-lines are written as noncommutative formulas. We show that the latter proof system is strictly stronger than resolution, polynomial calculus and polynomial calculus with resolution (PCR) and admits polynomial-size refutations for the pigeonhole principle and the Tseitin's formulas. We conclude by proposing an approach for establishing lower bounds on PC over ordered formulas proofs, and related systems, based on properties of lower bounds on noncommutative formulas. The motivation behind this work is developing techniques incorporating rank arguments (similar to those used in algebraic circuit complexity) for establishing lower bounds on propositional proofs.
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