Andrei Gagarin Gilbert Labelle Pierre Leroux The structure and unlabelled enumeration of toroidal graphs with no K<sub>3, 3</sub>'s. 69-76 2006 24 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/j.endm.2006.06.012 db/journals/endm/endm24.html#GagarinLL06