**矩阵:**由$n$行$m$列元素排成的$n \times m$阵列被称为$n \times m$的矩阵,记作$\mathbf{A}{n \times m}$或$\mathbf{A} = (a{ij}){n \times m}$,第$i$行第$j$列的元素记作$\mathbf{A}{ij}$或$a_{ij}$。所有$n \times m$阶矩阵构成的集合常记做$\mathcal{M}_{n \times m}(\R)$,特别地$n \times n$阶矩阵构成的集合可简称为$n$阶矩阵,简记为$\mathcal{M}_n(\R)$。
在$n$阶矩阵$\mathbf{A}$中,$\mathbf{A}_{ii}$被称为主对角线,只有主对角线非零的矩阵叫对角矩阵,记做$\mathbf{A} = \operatorname{diag} \{a_1, a_2, \dots, a_n\} = \begin{pmatrix} a_1 & 0 & \cdots & 0 \\ 0 & a_2 & \cdots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \cdots & a_n \end{pmatrix}$,特别地,主对角线全为$1$的矩阵叫做单位矩阵,$n$阶单位矩阵记作$\mathbf{I}_n = \begin{pmatrix} 1 & 0 & \cdots & 0 \\ 0 & 1 & \cdots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \cdots & 1 \end{pmatrix}$。
**矩阵加法:**设$\mathbf{A}, \mathbf{B} \in \mathcal{M}_{n \times m}(\R)$,则:
$$ (\mathbf{A} \pm \mathbf{B}){ij} = \mathbf{A}{ij} \pm \mathbf{B}{ij}, \forall 1 \leq i \leq n, 1 \leq j \leq m \\ \mathbf{A} \pm \mathbf{B} = \begin{pmatrix} \mathbf{A}{11} \pm \mathbf{B}{11} & \mathbf{A}{12} \pm \mathbf{B}{12} & \cdots & \mathbf{A}{1m} \pm \mathbf{B}{1m} \\ \mathbf{A}{21} \pm \mathbf{B}{21} & \mathbf{A}{22} \pm \mathbf{B}{22} & \cdots & \mathbf{A}{2m} \pm \mathbf{B}{2m} \\ \vdots & \vdots & \ddots & \vdots \\ \mathbf{A}{n1} \pm \mathbf{B}{n1} & \mathbf{A}{n2} \pm \mathbf{B}{n2} & \cdots & \mathbf{A}{nm} \pm \mathbf{B}_{nm} \\ \end{pmatrix} $$
**矩阵数乘:**设$\mathbf{A} \in \mathcal{M}_{n \times m}(\R), c \in \R$,则:
$$ (c\mathbf{A}){ij} = c\mathbf{A}{ij} \\ c\mathbf{A} = \begin{pmatrix} c\mathbf{A}{11} & c\mathbf{A}{12} & \cdots & c\mathbf{A}{1m} \\ c\mathbf{A}{21} & c\mathbf{A}{22} & \cdots & c\mathbf{A}{2m} \\ \vdots & \vdots & \ddots & \vdots \\ c\mathbf{A}{n1} & c\mathbf{A}{n2} & \cdots & c\mathbf{A}_{nm} \\ \end{pmatrix} $$