Maria Luisa Bonet Carlos Domingo Ricard Gavaldà Alexis Maciel Toniann Pitassi Non-Automatizability of Bounded-Depth Frege Proofs. 47-68 2004 13 Computational Complexity 1-2 http://dx.doi.org/10.1007/s00037-004-0183-5 db/journals/cc/cc13.html#BonetDGMP04