Eric Allender Michael Bauland Neil Immerman Henning Schnoor Heribert Vollmer The complexity of satisfiability problems: Refining Schaefer's theorem. 245-254 2009 75 J. Comput. Syst. Sci. 4 http://dx.doi.org/10.1016/j.jcss.2008.11.001 db/journals/jcss/jcss75.html#AllenderBISV09