Oleg A. Prokopyev Panos M. Pardalos On Approximability of Boolean Formula Minimization. 129-135 2004 8 J. Comb. Optim. 2 http://dx.doi.org/10.1023/B:JOCO.0000031414.39556.3a db/journals/jco/jco8.html#ProkopyevP04