TR16-098 Authors: Michael Forbes, Amir Shpilka, Iddo Tzameret, Avi Wigderson

Publication: 16th June 2016 14:33

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We give upper and lower bounds on the power of subsystems of the Ideal Proof System (IPS), the algebraic proof system recently proposed by Grochow and Pitassi, where the circuits comprising the proof come from various restricted algebraic circuit classes. This mimics an established research direction in the boolean setting for subsystems of Extended Frege proofs, where proof-lines are circuits from restricted boolean circuit classes. Except one, all of the subsystems considered in this paper can simulate the well-studied Nullstellensatz proof system, and prior to this work there were no known lower bounds when measuring proof size by the algebraic complexity of the polynomials (except with respect to degree, or to sparsity).

We give two general methods of converting certain algebraic lower bounds into proof complexity ones. Our methods require stronger notions of lower bounds, which lower bound a polynomial as well as an entire family of polynomials it defines. Our techniques are reminiscent of existing methods for converting boolean circuit lower bounds into related proof complexity results, such as feasible interpolation. We obtain the relevant types of lower bounds for a variety of classes (sparse polynomials, depth-3 powering formulas, read-once oblivious algebraic branching programs, and multilinear formulas), and infer the relevant proof complexity results. We complement our lower bounds by giving short refutations of the previously-studied subset-sum axiom using IPS subsystems, allowing us to conclude strict separations between some of these subsystems.

Our first method is a functional lower bound, a notion of Grigoriev and Razborov, which is a function $\hat{f}:\{0,1\}^n\to\mathbb{F}$ such that any polynomial $f$ agreeing with $\hat{f}$ on the boolean cube requires large algebraic circuit complexity. For our classes of interest, we develop functional lower bounds where $\hat{f}(\vec{x})$ equals $1/p(\vec{x})$ where $p$ is a constant-degree polynomial, which in turn yield corresponding IPS lower bounds for proving that $p$ is nonzero over the boolean cube. In particular, we show super-polynomial lower bounds for refuting variants of the subset-sum axiom in various IPS subsystems.

Our second method is to give lower bounds for multiples, that is, to give explicit polynomials whose all (nonzero) multiples require large algebraic circuit complexity. By extending known techniques, we are able to obtain such lower bounds for our classes of interest, which we then use to derive corresponding IPS lower bounds. Such lower bounds for multiples are of independent interest, as they have tight connections with the algebraic hardness versus randomness paradigm.