Chandra Chekuri Guy Even Anupam Gupta Danny Segev Set connectivity problems in undirected graphs and the directed Steiner network problem. 532-541 2008 SODA http://doi.acm.org/10.1145/1347082.1347141 conf/soda/2008 db/conf/soda/soda2008.html#ChekuriEGS08