Khaled M. Elbassioni Erik Krohn Domagoj Matijevic Julián Mestre Domagoj Severdija Improved Approximations for Guarding 1.5-Dimensional Terrains. 361-371 2009 STACS http://drops.dagstuhl.de/opus/volltexte/2009/1841 conf/stacs/2009 db/conf/stacs/stacs2009.html#ElbassioniKMMS09