Approximation and small depth Frege proofs

M. Ajtai (1988) recently proved that if, for some fixed d, every formula in a Frege proof of the propositional pigeonhole principle PHP_n has depth at most d, then the proof size is not less than any polynomial in n. By introducing the notion of an approximate proof the authors demonstrate how to eliminate the nonstandard model theory, including the nonconstructive use of the compactness theorem, from Ajtai´s lower bound. An approximate proof is one in which each inference is sound on a subset of the possible truth assignments-possibly a different subset for each inference. The authors also improve the lower bound, giving a specific superpolynomial function bounding the proof size from below