Maria Luisa Bonet Carlos Domingo Ricard Gavaldà Alexis Maciel Toniann Pitassi Non-Automatizability of Bounded-Depth Frege Proofs. 15-23 1999 IEEE Conference on Computational Complexity http://computer.org/proceedings/coco/0075/00750015abs.htm db/conf/coco/coco1999.html#BonetDGMP99