Classification of nonorientable regular embeddings of complete bipartite graphs

Abstract

A 2-cell embedding of a graph G into a closed (orientable or nonorientable) surface is called regular if its automorphism group acts regularly on the flags – mutually incident vertex–edge–face triples. In this paper, we classify the regular embeddings of complete bipartite graphs K n , n into nonorientable surfaces. Such a regular embedding of K n , n exists only when n is of the form n = 2 p 1 a 1 p 2 a 2 ⋯ p k a k where the p i are primes congruent to ±1 mod 8. In this case, up to isomorphism the number of those regular embeddings of K n , n is 2 k .