A note on regular sets in Cayley graphs

Abstract

A subset $R$ of the vertex set of a graph $\Gamma$ is said to be $(\kappa,\tau)$-regular if $R$ induces a $\kappa$-regular subgraph and every vertex outside $R$ is adjacent to exactly $\tau$ vertices in $R$. In particular, if $R$ is a $(\kappa,\tau)$-regular set of some Cayley graph on a finite group $G$, then $R$ is called a $(\kappa,\tau)$-regular set of $G$. Let $H$ be a non-trivial normal subgroup of $G$, and $\kappa$ and $\tau$ a pair of integers satisfying $0\leq\kappa\leq|H|-1$, $1\leq\tau\leq|H|$ and $\gcd(2,|H|-1)\mid\kappa$. It is proved that (i) if $\tau$ is even, then $H$ is a $(\kappa,\tau)$-regular set of $G$; (ii) if $\tau$ is odd, then $H$ is a $(\kappa,\tau)$-regular set of $G$ if and only if it is a $(0,1)$-regular set of $G$.