Christodoulos Fragoudakis Euripides Markou Stathis Zachos Maximizing the guarded boundary of an Art Gallery is APX-complete. 170-180 2007 38 Comput. Geom. 3 http://dx.doi.org/10.1016/j.comgeo.2006.12.001 db/journals/comgeo/comgeo38.html#FragoudakisMZ07