Eric Allender Michael Bauland Neil Immerman Henning Schnoor Heribert Vollmer The Complexity of Satisfiability Problems: Refining Schaefer's Theorem. 71-82 2005 conf/mfcs/2005 MFCS http://dx.doi.org/10.1007/11549345_8 db/conf/mfcs/mfcs2005.html#AllenderBISV05