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