S. Mishra Jaikumar Radhakrishnan Sivaramakrishnan Sivasubramanian On the Hardness of Approximating Minimum Monopoly Problems. 277-288 2002 conf/fsttcs/2002 FSTTCS http://link.springer.de/link/service/series/0558/bibs/2556/25560277.htm db/conf/fsttcs/fsttcs2002.html#MishraRS02