Conjugacy Class Equation

Theorem
Let $G$ be a group.

Let $\left|{G}\right|$ be the order of $G$.

Let $Z \left({G}\right)$ be the center of $G$.

Let $x \in G$.

Let $N_G \left({x}\right)$ be the normalizer of $x$ in $G$.

Let $\left[{G : N_G \left({x}\right)}\right]$ be the index of $N_G \left({x}\right)$ in $G$.

Let $m$ be the number of non-singleton conjugacy classes of $G$.

Then $\left|{G}\right| = \left|{Z \left({G}\right)}\right| + \sum_{j=1}^m \left[{G : N_G \left({x_j}\right)}\right]$.

Proof 1
From Conjugacy Classes of Center Elements are Singletons, all elements of $Z \left({G}\right)$ form their own singleton conjugacy classes.


 * If $G$ is abelian, then the result is certainly true, because then $Z \left({G}\right) = G$, from Center of Abelian Group is Whole Group and so there are as many conjugacy classes as there are elements in $Z \left({G}\right)$ and hence in $G$.


 * So, suppose $G$ is non-abelian. Thus $Z \left({G}\right) \ne G$ and therefore $G - Z \left({G}\right) \ne \varnothing$.

From Conjugacy Classes of Center Elements are Singletons, all the non-singleton conjugacy classes of $G$ are in $G - Z \left({G}\right)$. From the way the theorem has been worded, there are $m$ of them.

Let us choose one element from each of the non-singleton conjugacy classes and call them $x_1, x_2, \ldots, x_m$.

Thus, these conjugacy classes can be written $\mathrm{C}_{x_1}, \mathrm{C}_{x_2}, \ldots, \mathrm{C}_{x_m}$.

So $\left|{G - Z \left({G}\right)}\right| = \sum_{j=1}^m \left|{\mathrm{C}_{x_j}}\right|$,

or $\left|{G}\right| - \left|{Z \left({G}\right)}\right| = \sum_{j=1}^m \left|{\mathrm{C}_{x_j}}\right|$.

From Size of Conjugacy Class is Index of Normalizer, $\left|{\mathrm{C}_{x_j}}\right| = \left[{G : N_G \left({x_j}\right)}\right]$ and the result follows.

Proof 2
Let the distinct orbits of $G$ under the Conjugacy Action be $\operatorname{Orb} \left({x_1}\right), \operatorname{Orb} \left({x_2}\right), \ldots, \operatorname{Orb} \left({x_s}\right)$.

Then from the Partition Equation, $\left|{G}\right| = \left|{\operatorname{Orb} \left({x_1}\right)}\right| + \left|{\operatorname{Orb} \left({x_2}\right)}\right| + \cdots + \left|{\operatorname{Orb} \left({x_s}\right)}\right|$.

From the Orbit-Stabilizer Theorem, $\left|{\operatorname{Orb} \left({x_i}\right)}\right| \backslash \left|{G}\right|, i = 1, \ldots, s$.

The result follows from the definition of the Conjugacy Action.