Hardness Amplification in Proof Complexity

Abstract

We present a general method for converting any family of unsatisfiable CNF formulas that is hard for one of the simplest proof systems, tree resolution, into formulas that require large rank in any proof system that manipulates polynomials or polynomial threshold functions of degree at most k (known as Th(k) proofs). Such systems include Lovasz-Schrijver and Cutting Planes proof systems as well as their high degree analogues. These are based on analyzing two new proof systems, denoted by T^cc(k) and R^cc(k). The proof lines of T^cc(k) are arbitrary Boolean functions, each of which can be evaluated by an efficient k-party randomized communication protocol. They include Th{k-1} proofs as a special case. R^cc(k) proofs are stronger and only require that each inference be locally checkable by an efficient k-party randomized communication protocol. Our main results are the following: (1) When k is O(loglogn), for any unsatisfiable CNF formula F requiring resolution rank r, there is a related CNF formula G=Lift_k(F) requiring refutation rank r^Omega(1/k) log^O(1) n in all R^cc(k) systems. (2) There are strict hierarchies for T^cc(k) and R^cc(k) systems with respect to k when k is O(loglogn in that there are unsatisfiable CNF formulas requiring large rank R^cc(k) refutations but having log^O(1) n rank Th(k) refutations. (3) When k is O(loglogn) there are 2^(n^Omega(1/k)) lower bounds on the size of tree-like T^cc(k) refutations for large classes of lifted CNF formulas. (4) A general method for producing integrality gaps for low rank R^cc(2) inference (and hence Cutting Planes and Th(1) inference) based on related gaps for low rank resolution. These gaps are optimal for MAX-2t-SAT.