Arc-transitive cubic abelian bi-Cayley graphs and BCI-graphs

István Kovács, Hiroki Koike


A finite simple graph is called a bi-Cayley graph over a group $H$ if it has a semiregular automorphism group, isomorphic to $H,$ which has two orbits on the vertex set. Cubic vertex-transitive bi-Cayley graphs over abelian groups have been classified recently by Feng and Zhou (Europ. J. Combin. 36 (2014), 679--693). In this paper we consider the latter class of graphs and select those in the class which are also arc-transitive. Furthermore, such a graph is called $0$-type when it is bipartite, and the bipartition classes are equal to the two orbits of the respective semiregular automorphism group. A $0$-type graph can be represented as the graph $\bc(H,S),$ where $S$ is a subset of $H,$ the vertex set of which consists of two copies of $H,$ say $H_0$ and $H_1,$ and the edge set is $\{ \{h_0,g_1\} : h,g \in H, g h^{-1} \in S\}$. A bi-Cayley graph $\bc(H,S)$ is called a BCI-graph if for any bi-Cayley graph $\bc(H,T),$ $\bc(H,S) \cong \bc(H,T)$ implies that $T = h S^\alpha$ for some $h \in H$ and $\alpha \in \aut(H)$. It is also shown that every cubic connected arc-transitive $0$-type bi-Cayley graph over an abelian group is a BCI-graph.

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