Conjugacy Class of Element of Center is Singleton/Corollary
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Corollary to Conjugacy Class of Element of Center is Singleton
Let $G$ be a group.
Let $\map Z G$ denote the center of $G$.
The number of single-element conjugacy classes of $G$ is the order of $\map Z G$ and divides $\order G$.
Proof
From Conjugacy Class of Element of Center is Singleton, each of the singleton conjugacy classes consists of one of the elements of $\map Z G$.
By Center of Group is Subgroup, $\map Z G$ is a subgroup of $G$.
It follows from Lagrange's Theorem that the number of these divides the order of $G$.
$\blacksquare$