Chandrashekhar Nagarajan Yogeshwer Sharma David P. Williamson Approximation Algorithms for Prize-Collecting Network Design Problems with General Connectivity Requirements. 174-187 2008 WAOA http://dx.doi.org/10.1007/978-3-540-93980-1_14 conf/waoa/2008 db/conf/waoa/waoa2008.html#NagarajanSW08