On 2-Fold Covers of Graphs

A regular covering projection \p\colon \tX \to X of connected graphs is $G$-admissible if $G$ lifts along \p. Denote by \tG the lifted group, and let \CT(\p) be the group of covering transformations. The projection is called $G$-split whenever the extension \CT(\p) \to \tG \to G splits. In this paper, split 2-covers are considered. Supposing that $G$ is transitive on $X$, a $G$-split cover is said to be $G$-split-transitive if all complements \bG \cong G of \CT(\p) within \tG are transitive on \tX; it is said to be $G$-split-sectional whenever for each complement \bG there exists a \bG-invariant section of \p; and it is called $G$-split-mixed otherwise. It is shown, when $G$ is an arc-transitive group, split-sectional and split-mixed 2-covers lead to canonical double covers. For cubic symmetric graphs split 2-cover are necessarily cannonical double covers when $G$ is 1- or 4-regular. In all other cases, that is, if $G$ is $s$-regular, $s=2,3$ or 5, a necessary and sufficient condition for the existence of a transitive complement \bG is given, and an infinite family of split-transitive 2-covers based on the alternating groups of the form $A_{12k+10}$ is constructed. Finally, chains of consecutive 2-covers, along which an arc-transitive group $G$ has successive lifts, are also considered. It is proved that in such a chain, at most two projections can be split. Further, it is shown that, in the context of cubic symmetric graphs, if exactly two of them are split, then one is split-transitive and the other one is either split-sectional or split-mixed.Comment: 18 pages, 3 figure

On cubic symmetric non-Cayley graphs with solvable automorphism groups

It was proved in [Y.-Q. Feng, C. H. Li and J.-X. Zhou, Symmetric cubic graphs with solvable automorphism groups, {\em European J. Combin.} {\bf 45} (2015), 1-11] that a cubic symmetric graph with a solvable automorphism group is either a Cayley graph or a $2$-regular graph of type $2^2$, that is, a graph with no automorphism of order $2$ interchanging two adjacent vertices. In this paper an infinite family of non-Cayley cubic $2$-regular graphs of type $2^2$ with a solvable automorphism group is constructed. The smallest graph in this family has order 6174.Comment: 8 page

Permutation groups and normal subgroups

Various descending chains of subgroups of a finite permutation group can be used to define a sequence of `basic' permutation groups that are analogues of composition factors for abstract finite groups. Primitive groups have been the traditional choice for this purpose, but some combinatorial applications require different kinds of basic groups, such as quasiprimitive groups, that are defined by properties of their normal subgroups. Quasiprimitive groups admit similar analyses to primitive groups, share many of their properties, and have been used successfully, for example to study $s$-arc transitive graphs. Moreover investigating them has led to new results about finite simple groups

On the orders of arc-transitive graphs

A graph is called {\em arc-transitive} (or {\em symmetric}) if its automorphism group has a single orbit on ordered pairs of adjacent vertices, and 2-arc-transitive its automorphism group has a single orbit on ordered paths of length 2. In this paper we consider the orders of such graphs, for given valency. We prove that for any given positive integer $k$, there exist only finitely many connected 3-valent 2-arc-transitive graphs whose order is $kp$ for some prime $p$, and that if $d\ge 4$, then there exist only finitely many connected $d$-valent 2-arc-transitive graphs whose order is $kp$ or $kp^2$ for some prime $p$. We also prove that there are infinitely many (even) values of $k$ for which there are only finitely many connected 3-valent symmetric graphs of order $kp$ where $p$ is prime

Constructing 2-Arc-Transitive Covers of Hypercubes

We introduce the notion of a symmetric basis of a vector space equipped with a quadratic form, and provide a sufficient and necessary condition for the existence to such a basis. Symmetric bases are then used to study Cayley graphs of certain extraspecial 2-groups of order 2^{2r+1} (r\geq 1), which are further shown to be normal Cayley graphs and 2-arc-transitive covers of 2r-dimensional hypercubes

Semisymmetric elementary abelian covers of the M\"obius-Kantor graph

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 M\"{o}bius-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 $p \equiv 5,9,13,17,21 (\mod 24)$ and $p \equiv 1 (\mod 24)$, respectively, and to $(p+1)/4$ and $1+ (p+1)/4$ in cases $p \equiv 3,7,11,15,23 (\mod 24)$ and $p \equiv 19 (\mod 24)$, respectively. For each such covering projection the voltage rules generating the corresponding covers are displayed explicitly.Comment: 23 pages, 8 figure

Four-Valent Oriented Graphs of Biquasiprimitive Type

Let $\mathcal{OG}(4)$ denote the family of all graph-group pairs $(\Gamma,G)$ where $\Gamma$ is 4-valent, connected and $G$-oriented ($G$-half-arc-transitive). Using a novel application of the structure theorem for biquasiprimitive permutation groups of the second author, we produce a description of all pairs $(\Gamma, G) \in\mathcal{OG}(4)$ for which every nontrivial normal subgroup of $G$ has at most two orbits on the vertices of $\Gamma$. In particular we show that $G$ has a unique minimal normal subgroup $N$ and that $N \cong T^k$ for a simple group $T$ and $k\in \{1,2,4,8\}$. This provides a crucial step towards a general description of the long-studied family $\mathcal{OG}(4)$ in terms of a normal quotient reduction. We also give several methods for constructing pairs $(\Gamma, G)$ of this type and provide many new infinite families of examples, covering each of the possible structures of the normal subgroup $N$

Graphs that contain multiply transitive matchings

Let $\Gamma$ be a finite, undirected, connected, simple graph. We say that a matching $\mathcal{M}$ is a \textit{permutable $m$-matching} if $\mathcal{M}$ contains $m$ edges and the subgroup of $\text{Aut}(\Gamma)$ that fixes the matching $\mathcal{M}$ setwise allows the edges of $\mathcal{M}$ to be permuted in any fashion. A matching $\mathcal{M}$ is \textit{2-transitive} if the setwise stabilizer of $\mathcal{M}$ in $\text{Aut}(\Gamma)$ can map any ordered pair of distinct edges of $\mathcal{M}$ to any other ordered pair of distinct edges of $\mathcal{M}$. We provide constructions of graphs with a permutable matching; we show that, if $\Gamma$ is an arc-transitive graph that contains a permutable $m$-matching for $m \ge 4$, then the degree of $\Gamma$ is at least $m$; and, when $m$ is sufficiently large, we characterize the locally primitive, arc-transitive graphs of degree $m$ that contain a permutable $m$-matching. Finally, we classify the graphs that have a $2$-transitive perfect matching and also classify graphs that have a permutable perfect matching.Comment: to appear in European Journal of Combinatoric

Vertex-transitive Haar graphs that are not Cayley graphs

In a recent paper (arXiv:1505.01475 ) Est\'elyi and Pisanski raised a question whether there exist vertex-transitive Haar graphs that are not Cayley graphs. In this note we construct an infinite family of trivalent Haar graphs that are vertex-transitive but non-Cayley. The smallest example has 40 vertices and is the well-known Kronecker cover over the dodecahedron graph $G(10,2)$, occurring as the graph $40$ in the Foster census of connected symmetric trivalent graphs.Comment: 9 pages, 2 figure

On quartic half-arc-transitive metacirculants

Following Alspach and Parsons, a {\em metacirculant graph} is a graph admitting a transitive group generated by two automorphisms $\rho$ and $\sigma$, where $\rho$ is $(m,n)$-semiregular for some integers $m \geq 1$, $n \geq 2$, and where $\sigma$ normalizes $\rho$, cyclically permuting the orbits of $\rho$ in such a way that $\sigma^m$ has at least one fixed vertex. A {\em half-arc-transitive graph} is a vertex- and edge- but not arc-transitive graph. In this article quartic half-arc-transitive metacirculants are explored and their connection to the so called tightly attached quartic half-arc-transitive graphs is explored. It is shown that there are three essentially different possibilities for a quartic half-arc-transitive metacirculant which is not tightly attached to exist. These graphs are extensively studied and some infinite families of such graphs are constructed.Comment: 31 pages, 2 figure