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

TL;DR

This paper establishes exponential lower bounds on the number of non-isomorphic $k$-gonal biembeddings of complete multipartite graphs into orientable surfaces, using Heffer arrays and combinatorial constructions.

Contribution

It introduces a method to derive exponential quantities of distinct graph embeddings from single Heffer arrays, expanding known bounds for infinitely many parameters.

Findings

Exponential lower bounds on non-isomorphic embeddings are proven.

Construction methods for embeddings of specific complete multipartite graphs are provided.

Results apply to infinitely many values of $k$ and $v$, with explicit bounds.

Abstract

This article aims to provide exponential lower bounds on the number of non-isomorphic $k$-gonal 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 Yazici in 2020, we obtain that, for infinitely many values of $k$ and $v$, there are at least $k^{\frac{k}{2}+o(k)} \cdot 2^{v\cdot \frac{H(1/4)}{(2k)^2}+o(v)}$ non-isomorphic $k$-gonal biembeddings of $K_v$, where $H(\cdot)$ is the binary entropy. Moreover about the embeddings of $K_{\frac{v}{t}\times t}$, for $t\in\{1,2,k\}$, we provide a construction of $2^{v\cdot \frac{H(1/4)}{2k(k-1)}+o(v,k)}$ non-isomorphic $k$-gonal biembeddings whenever $k$ is odd and $v$ belongs to a wide infinite family of values.