Luerbio Faria Celina M. Herrera de Figueiredo Candido Ferreira Xavier de Mendonça Neto On the complexity of the approximation of nonplanarity parameters for cubic graphs. 18-21 2001 7 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/S1571-0653(04)00214-8 db/journals/endm/endm7.html#FariaFN01