Marta Arias Aaron Feigelson Roni Khardon Rocco A. Servedio Polynomial certificates for propositional classes. 816-834 2006 204 Inf. Comput. 5 http://dx.doi.org/10.1016/j.ic.2006.03.001 db/journals/iandc/iandc204.html#AriasFKS06