Kyung-Yong Chwa Byung-Cheol Jo Christian Knauer Esther Moet René van Oostrum Chan-Su Shin Guarding Art Galleries by Guarding Witnesses. 205-226 2006 16 Int. J. Comput. Geometry Appl. 2-3 http://dx.doi.org/10.1142/S0218195906002002 db/journals/ijcga/ijcga16.html#ChwaJKMOS06