\begin{equation*} A_{m,n} = \begin{pmatrix} a_{1,1} & a_{1,2} & \cdots & a_{1,n} \\ a_{2,1} & a_{2,2} & \cdots & a_{2,n} \\ \vdots & \vdots & \ddots & \vdots \\ a_{m,1} & a_{m,2} & \cdots & a_{m,n} \end{pmatrix} \end{equation*}
\begin{bmatrix} 1 & 2 & 3 \\ a & b & c \\ a & b & c \\ \end{bmatrix}
Trying to typeset an inline matrix here $\begin{pmatrix} a & b\\ c & d \end{pmatrix}$ but it looks too big, so let's try $\big(\begin{smallmatrix} a & b\\ c & d \end{smallmatrix}\big)$ instead.
$\left(\begin{array}{cc} a & b \\ c & d \end{array}\right)$
