Ralf Klasing Euripides Markou Tomasz Radzik Fabiano Sarracco Hardness and approximation results for Black Hole Search in arbitrary networks. 201-221 2007 384 Theor. Comput. Sci. 2-3 http://dx.doi.org/10.1016/j.tcs.2007.04.024 db/journals/tcs/tcs384.html#KlasingMRS07