1. Gardner, M.: Mathematical games. Scientific American Feb, 14–19 (1908)
2. Harary, F.: Graph Theory, Reading: Addison-Wesley 1969
3. Hartsfield, N.: The toroidal splitting number of the complete graphK n Ph.D. Dissertation, University of California, Santa Cruz
4. Heawood, P.J.: Map color theorem. Q.J. Math24, 332–338 (1890)
5. Jackson, B., Ringel, G.: Solution of Heawood's empire problem in the plane. J.f.d. reine u. angew. Math.347, 146–153 (1984)