Planar graphs on the projective plane
It is shown that embeddings of planar graphs in the projective plane necessarily have very specific structure, which leads to an indirect characterization of projective-planar graphs whose duals are planar. Also Whitney's planar 2-switching theorem is generalized: Any two embeddings of a planar graph in the projective plane can be obtained from each other by means of simple local reembeddings (similar to Whitney's switchings).
scientific article; zbMATH DE number 858108