Stephan Eidenbenz Christoph Stamm Peter Widmayer Positioning Guards at Fixed Height Above a Terrain - An Optimum Inapproximability Result. 187-198 http://link.springer.de/link/service/series/0558/bibs/1461/14610187.htm 1998 conf/esa/1998 ESA db/conf/esa/esa98.html#EidenbenzSW98