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Published online by Cambridge University Press: 12 May 2021
Given a finite group G with a normal subgroup N, the simple graph $\Gamma _{\textit {G}}( \textit {N} )$ is a graph whose vertices are of the form
$|x^G|$, where
$x\in {N\setminus {Z(G)}}$ and
$x^G$ is the G-conjugacy class of N containing the element x. Two vertices
$|x^G|$ and
$|y^G|$ are adjacent if they are not coprime. We prove that, if
$\Gamma _G(N)$ is a connected incomplete regular graph, then
$N= P \times {A}$ where P is a p-group, for some prime p,
$A\leq {Z(G)}$ and
$\textbf {Z}(N)\not = N\cap \textbf {Z}(G)$.
The research of the second author was in part supported by a grant from IPM (No. 1400200028).