Euripides Markou Stathis Zachos Christodoulos Fragoudakis Maximizing the Guarded Boundary of an Art Gallery Is APX-Complete. 24-35 2003 conf/ciac/2003 CIAC http://link.springer.de/link/service/series/0558/bibs/2653/26530024.htm db/conf/ciac/ciac2003.html#MarkouZF03