Non-zero sum Heffter arrays and their applications

In this paper we introduce a new class of partially filled arrays that, as Heffter arrays, are related to difference families, graph decompositions and biembeddings. A non-zero sum Heffter array $\mathrm{N}\mathrm{H}(m,n; h,k)$ is an $m \times n$ p. f. array with entries in $\mathbb{Z}_{2nk+1}$ such that: each row contains $h$ filled cells and each column contains $k$ filled cells; for every $x\in \mathbb{Z}_{2nk+1}\setminus\{0\}$, either $x$ or $-x$ appears in the array; the sum of the elements in every row and column is different from $0$ (in $\mathbb{Z}_{2nk+1}$). Here first we explain the connections with relative difference families and with path decompositions of the complete multipartite graph. Then we present a complete solution for the existence problem and a constructive complete solution for the square case and for the rectangular case with no empty cells when the additional, very restrictive, property of "globally simple" is required. Finally, we show how these arrays can be used to construct biembeddings of complete graphs.

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