## Section13.2The Class Equation

Let $X$ be a finite $G$-set and $X_G$ be the set of fixed points in $X\text{;}$ that is,

\begin{equation*} X_G = \{ x \in X : gx = x \text{ for all } g \in G \}\text{.} \end{equation*}

Since the orbits of the action partition $X\text{,}$

\begin{equation*} |X| = |X_G| + \sum_{i = k}^n |{\mathcal O}_{x_i}|\text{,} \end{equation*}

where $x_k, \ldots, x_n$ are representatives from the distinct nontrivial orbits of $X\text{.}$

Now consider the special case in which $G$ acts on itself by conjugation, $(g,x) \mapsto gxg^{-1}\text{.}$ The center of $G\text{,}$

\begin{equation*} Z(G) = \{x : xg = gx \text{ for all } g \in G \}\text{,} \end{equation*}

is the set of points that are fixed by conjugation. The nontrivial orbits of the action are called the conjugacy classes of $G\text{.}$ If $x_1, \ldots, x_k$ are representatives from each of the nontrivial conjugacy classes of $G$ and $|{\mathcal O}_{x_1}| = n_1, \ldots, |{\mathcal O}_{x_k}| = n_k\text{,}$ then

\begin{equation*} |G| = |Z(G)| + n_1 + \cdots + n_k\text{.} \end{equation*}

The stabilizer subgroups of each of the $x_i$'s, $C(x_i) = \{ g \in G: g x_i = x_i g \}\text{,}$ are called the centralizer subgroups of the $x_i$'s. From TheoremĀ 13.11, we obtain the class equation:

\begin{equation*} |G| = |Z(G)| + [G: C(x_1) ] + \cdots + [ G: C(x_k)]\text{.} \end{equation*}

One of the consequences of the class equation is that the order of each conjugacy class must divide the order of $G\text{.}$

###### Example13.12.

It is easy to check that the conjugacy classes in $S_3$ are the following:

\begin{equation*} \{ (1) \}, \quad \{ (123), (132) \}, \quad \{(12), (13), (23) \}\text{.} \end{equation*}

The class equation is $6 = 1+2+3\text{.}$

###### Example13.13.

The center of $D_4$ is $\{ (1), (13)(24) \}\text{,}$ and the conjugacy classes are

\begin{equation*} \{ (13), (24) \}, \quad \{ (1432), (1234) \}, \quad \{ (12)(34), (14)(23) \}\text{.} \end{equation*}

Thus, the class equation for $D_4$ is $8 = 2 + 2 + 2 + 2\text{.}$

###### Example13.14.

For $S_n$ it takes a bit of work to find the conjugacy classes. We begin with cycles. Suppose that $\sigma = ( a_1, \ldots, a_k)$ is a cycle and let $\tau \in S_n\text{.}$ By TheoremĀ 11.23,

\begin{equation*} \tau \sigma \tau^{-1} = ( \tau( a_1), \ldots, \tau(a_k))\text{.} \end{equation*}

Consequently, any two cycles of the same length are conjugate. Now let $\sigma = \sigma_1 \sigma_2 \cdots \sigma_r$ be a cycle decomposition, where the length of each cycle $\sigma_i$ is $r_i\text{.}$ Then $\sigma$ is conjugate to every other $\tau \in S_n$ whose cycle decomposition has the same lengths.

The number of conjugate classes in $S_n$ is the number of ways in which $n$ can be partitioned into sums of positive integers. In the case of $S_3$ for example, we can partition the integer $3$ into the following three sums:

\begin{align*} 3 & = 1 + 1 + 1\\ 3 & = 1 + 2\\ 3 & = 3; \end{align*}

therefore, there are three conjugacy classes. The problem of finding the number of such partitions for any positive integer $n$ is what computer scientists call NP-complete. This effectively means that the problem cannot be solved for a large $n$ because the computations would be too time-consuming for even the largest computer.

###### Proof.

We apply the class equation

\begin{equation*} |G| = |Z(G)| + n_1 + \cdots + n_k\text{.} \end{equation*}

Since each $n_i \gt 1$ and $n_i \mid |G|\text{,}$ it follows that $p$ must divide each $n_i\text{.}$ Also, $p \mid |G|\text{;}$ hence, $p$ must divide $|Z(G)|\text{.}$ Since the identity is always in the center of $G\text{,}$ $|Z(G)| \geq 1\text{.}$ Therefore, $|Z(G)| \geq p\text{,}$ and there exists some $g \in Z(G)$ such that $g \neq 1\text{.}$

###### Proof.

By TheoremĀ 13.15, $|Z(G)| = p$ or $p^2\text{.}$ If $|Z(G)| = p^2\text{,}$ then we are done. Suppose that $|Z(G)| = p\text{.}$ Then $Z(G)$ and $G / Z(G)$ both have order $p$ and must both be cyclic groups. Choosing a generator $aZ(G)$ for $G / Z(G)\text{,}$ we can write any element $gZ(G)$ in the quotient group as $a^m Z(G)$ for some integer $m\text{;}$ hence, $g = a^m x$ for some $x$ in the center of $G\text{.}$ Similarly, if $hZ(G) \in G / Z(G)\text{,}$ there exists a $y$ in $Z(G)$ such that $h = a^n y$ for some integer $n\text{.}$ Since $x$ and $y$ are in the center of $G\text{,}$ they commute with all other elements of $G\text{;}$ therefore,

\begin{equation*} gh = a^m x a^n y = a^{m+n} x y = a^n y a^m x = hg\text{,} \end{equation*}

and $G$ must be abelian.