The following publications are possibly variants of this publication:
- Approximation and Small Depth Frege ProofsStephen Bellantoni, Toniann Pitassi, Alasdair Urquhart. cocoon 1991: 367-390
- Approximation and Small-Depth Frege ProofsStephen Bellantoni, Toniann Pitassi, Alasdair Urquhart. SICOMP, 21(6):1161-1179, 1992.
- Non-Automatizability of Bounded-Depth Frege ProofsMaria Luisa Bonet, Carlos Domingo, Ricard Gavaldà , Alexis Maciel, Toniann Pitassi. cocoon 1999: 15-23 [doi]
- Non-Automatizability of Bounded-Depth Frege ProofsMaria Luisa Bonet, Carlos Domingo, Ricard Gavaldà , Alexis Maciel, Toniann Pitassi. cc, 13(1-2):47-68, 2004. [doi]