Philipp Hertel Toniann Pitassi Exponential Time/Space Speedups for Resolution and the PSPACE-completeness of Black-White Pebbling. 137-149 2007 FOCS http://doi.ieeecomputersociety.org/10.1109/FOCS.2007.25 conf/focs/2007 db/conf/focs/focs2007.html#HertelP07