On the number of non-isomorphic (simple) k-gonal biembeddings of complete multipartite graphs

This article aims to provide exponential lower bounds on the number of non-isomorphic k-gonal face-2-colourable embeddings (sometimes called, with abuse of notation, biembeddings) of the complete multipartite graph into orientable surfaces. For this purpose, we use the concept, introduced by Archdeacon in 2015, of Heffer array and its relations with graph embeddings. In particular we show that, under certain hypotheses, from a single Heffter array, we can obtain an exponential number of distinct graph embeddings. Exploiting this idea starting from the arrays constructed by Cavenagh, Donovan and Yazıcı in 2020, we obtain that, for infinitely many values of k and v, there are at least kk/2 + o(k) ⋅ 2v ⋅ H(1/4)/(2k)^2 + o(v) non-isomorphic k-gonal face-2-colourable embeddings of Kv, where H(⋅) is the binary entropy. Moreover about the embeddings of Kv/t × t, for t ∈ {1, 2, k}, we provide a construction of 2v ⋅ H(1/4)/2k(k−1) + o(v,k) non-isomorphic k-gonal face-2-colourable embeddings whenever k is odd and v belongs to a wide infinite family of values.