Lower bounds on Hilbert's Nullstellensatz and propositional proofs

The weak form of the Hilbert's Nullstellensatz says that a system of algebraic equations over a field, Q/sub i/(x~)=0, does not have a solution in the algebraic closure iff 1 is in the ideal generated by the polynomials Q/sub i/(x~). We shall prove a lower bound on the degrees of polynomials P/sub i/(x~) such that /spl Sigma//sub i/ P/sub i/(x~)Q/sub i/(x~)=1. This result has the following application. The modular counting principle states that no finite set whose cardinality is not divisible by q can be partitioned into q-element classes. For each fixed cardinality N, this principle can be expressed as a propositional formula Count/sub q//sup N/. Ajtai (1988) proved recently that, whenever p, q are two different primes, the propositional formulas Count/sub q//sup qn+1/ do not have polynomial size, constant-depth Frege proofs from instances of Count/sub p//sup m/, m/spl ne/0 (mod p). We give a new proof of this theorem based on the lower bound for the Hilbert's Nullstellensatz. Furthermore our technique enables us to extend the independence results for counting principles to composite numbers p and q. This results in an exact characterization of when Count/sub q/ can be proven efficiently from Count/sub p/, for all p and q.< >