Refael Hassin Asaf Levin A Better-Than-Greedy Approximation Algorithm for the Minimum Set Cover Problem. 189-200 2005 35 SIAM J. Comput. 1 http://epubs.siam.org/SICOMP/volume-35/art_44475.html db/journals/siamcomp/siamcomp35.html#HassinL05