Marc Denecker Victor W. Marek Miroslaw Truszczynski Ultimate approximation and its application in nonmonotonic knowledge representation systems. 84-121 2004 192 Inf. Comput. 1 http://dx.doi.org/10.1016/j.ic.2004.02.004 db/journals/iandc/iandc192.html#DeneckerMT04