1. S. Alpert, Current maps and nonorientable graph inbeddings, in preparation.
2. The genus of the n-cube;Beineke;Canad. J. Math.,1965
3. Covering spaces with singularities;Fox,1957
4. Voltage graphs;Gross;Discrete Math.,1974
5. Branched coverings of graph imbeddings;Gross;Bull. Amer. Math. Soc.,1973