Lichen Bao Sergey Bereg Ovidiu Daescu Simeon C. Ntafos Junqiang Zhou On Some City Guarding Problems. 600-610 2008 COCOON http://dx.doi.org/10.1007/978-3-540-69733-6_59 conf/cocoon/2008 db/conf/cocoon/cocoon2008.html#BaoBDNZ08