Michael Alekhnovich Samuel R. Buss Shlomo Moran Toniann Pitassi Minimum Propositional Proof Length Is NP-Hard to Linearly Approximate. 171-191 2001 66 J. Symb. Log. 1 db/journals/jsyml/jsyml66.html#AlekhnovichBMP01 http://projecteuclid.org/euclid.jsl/1183746365