Guy Even Joseph Naor Baruch Schieber Leonid Zosin Approximating Minimum Subset Feedback Sets in Undirected Graphs with Applications. 255-267 2000 13 SIAM J. Discrete Math. 2 http://epubs.siam.org/sam-bin/dbq/article/29187 db/journals/siamdm/siamdm13.html#EvenNSZ00