Semisymmetric elementary abelian covers of the Möbius-Kantor graph

Abstract

Let $\p_N \colon \tX \to X$ be a regular covering projection of connected graphs with the group of covering transformations isomorphic to $N$. If $N$ is an elementary abelian $p$-group, then the projection $\p_N$ is called $p$-elementary abelian. The projection $\p_N$ is vertex-transitive (edge-transitive) if some vertex-transitive (edge-transitive) subgroup of $\Aut X$ lifts along $\p_N$, and semisymmetric if it is edge- but not vertex-transitive. The projection $\p_N$ is minimal semisymmetric if $p_N$ cannot be written as a composition $\p_N = \p \circ \p_M$ of two (nontrivial) regular covering projections, where $\p_M$ is semisymmetric. Finding elementary abelian covering projections can be grasped combinatorially via a linear representation of automorphisms acting on the first homology group of the graph. The method essentially reduces to finding invariant subspaces of matrix groups over prime fields (see {\em J. Algebr. Combin.}, {\bf 20} (2004), 71--97). In this paper, all pairwise nonisomorphic minimal semisymmetric elementary abelian regular covering projections of the Möbius-Kantor graph, the Generalized Petersen graph $\GP(8,3)$, are constructed. No such covers exist for $p=2$. Otherwise, the number of such covering projections is equal to $(p-1)/4$ and $1+(p-1)/4$ in cases $p\equiv 5,9,13,17,21(\mathrm{mod}24)$ and $p\equiv 1(\mathrm{mod}24)$, respectively, and to $(p+1)/4$ and $1+(p+1)/4$ in cases $p\equiv 3,7,11,15,23(\mathrm{mod}24)$ and $p\equiv 19(\mathrm{mod}24)$, respectively. For each such covering projection the voltage rules generating the corresponding covers are displayed explicitly.

23 pages, 8 figures