Russell Impagliazzo Nathan Segerlind Constant-depth Frege systems with counting axioms polynomially simulate Nullstellensatz refutations. 199-218 2006 7 ACM Trans. Comput. Log. 2 http://doi.acm.org/10.1145/1131313.1131314 db/journals/tocl/tocl7.html#ImpagliazzoS06