Hans L. Bodlaender Alexander Grigoriev Nadejda V. Grigorieva Albert Hendriks The Valve Location Problem in Simple Network Topologies. 55-65 2008 WG http://dx.doi.org/10.1007/978-3-540-92248-3_6 conf/wg/2008 db/conf/wg/wg2008.html#BodlaenderGGH08