Rafael Ayala Eladio Domínguez Angel R. Francés Antonio Quintero Weak lighting functions and strong 26-surfaces. 29-66 2002 283 Theor. Comput. Sci. 1 db/journals/tcs/tcs283.html#AyalaDFQ02 http://dx.doi.org/10.1016/S0304-3975(01)00051-2