# 置换矩阵

## 严格定义

$\pi : \lbrace 1, \ldots, n \rbrace \to \lbrace 1, \ldots, n \rbrace$

$\begin{pmatrix} 1 & 2 & \cdots & n \\ \pi(1) & \pi(2) & \cdots & \pi(n) \end{pmatrix},$

$P_\pi = \begin{bmatrix} \mathbf e_{\pi(1)} \\ \mathbf e_{\pi(2)} \\ \vdots \\ \mathbf e_{\pi(n)} \end{bmatrix},$

$I = \begin{bmatrix} \mathbf e_1 \\ \mathbf e_2 \\ \vdots \\ \mathbf e_n\end{bmatrix}$

## 性质

$P_{\pi} P_{\sigma} = P_{\pi \circ \sigma}$

$P_{\pi}^{-1} = P_{\pi^{-1}} = P_{\pi}^{T}$

$P_\pi \mathbf{g} = \begin{bmatrix} \mathbf{e}_{\pi(1)} \\ \mathbf{e}_{\pi(2)} \\ \vdots \\ \mathbf{e}_{\pi(n)} \end{bmatrix} \begin{bmatrix} g_1 \\ g_2 \\ \vdots \\ g_n \end{bmatrix} = \begin{bmatrix} g_{\pi(1)} \\ g_{\pi(2)} \\ \vdots \\ g_{\pi(n)} \end{bmatrix}.$

$\mathbf{h}P_\pi = \begin{bmatrix} h_1 \; h_2 \; \dots \; h_n \end{bmatrix} \begin{bmatrix} \mathbf{e}_{\pi(1)} \\ \mathbf{e}_{\pi(2)} \\ \vdots \\ \mathbf{e}_{\pi(n)} \end{bmatrix} = \begin{bmatrix} h_{\pi(1)} \; h_{\pi(2)} \; \dots \; h_{\pi(n)} \end{bmatrix}$

## 置换矩阵与置换

Snn次对称群，由于n置换一共有n! 个，n阶的置换矩阵也有n! 个。这n! 个置换矩阵构成一个关于矩阵乘法的。这个群的单位元就是单位矩阵。设A是所有n阶的置换矩阵的集合。映射Sn → A ⊂ GL(n, Z2)是一个群的忠实表示

## 例子

$P_\pi = \begin{bmatrix} \mathbf{e}_{\pi(1)} \\ \mathbf{e}_{\pi(2)} \\ \mathbf{e}_{\pi(3)} \\ \mathbf{e}_{\pi(4)} \\ \mathbf{e}_{\pi(5)} \end{bmatrix} = \begin{bmatrix} \mathbf{e}_{1} \\ \mathbf{e}_{4} \\ \mathbf{e}_{2} \\ \mathbf{e}_{5} \\ \mathbf{e}_{3} \end{bmatrix} = \begin{bmatrix} 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 & 0 \end{bmatrix}.$

$P_\pi \mathbf{g} = \begin{bmatrix} \mathbf{e}_{\pi(1)} \\ \mathbf{e}_{\pi(2)} \\ \mathbf{e}_{\pi(3)} \\ \mathbf{e}_{\pi(4)} \\ \mathbf{e}_{\pi(5)} \end{bmatrix} \begin{bmatrix} g_1 \\ g_2 \\ g_3 \\ g_4 \\ g_5 \end{bmatrix} = \begin{bmatrix} g_1 \\ g_4 \\ g_2 \\ g_5 \\ g_3 \end{bmatrix}.$