Toshihiro Fujito On Combinatorial Approximation of Covering 0-1 Integer Programs and Partial Set Cover. 439-452 2004 8 J. Comb. Optim. 4 http://www.springerlink.com/index/10.1007/s10878-004-4836-8 db/journals/jco/jco8.html#Fujito04