Jones, G. A.; Thornton, J. S., Operations on maps, and outer automorphisms, Journal of Combinatorial Theory, Series B, 1983, 35 (2): 93–103, MR 0733017, doi:10.1016/0095-8956(83)90065-5
Jones, G. A.; Thornton, J. S., Operations on maps, and outer automorphisms, Journal of Combinatorial Theory, Series B, 1983, 35 (2): 93–103, MR 0733017, doi:10.1016/0095-8956(83)90065-5