TeX source:
A=\boldsymbol{A}=\underline {A}=\begin{pmatrix}a_{11} & a_{12} & a_{13} & \cdots & a_{1n}\\a_{21} & a_{22} & a_{23} & \cdots & a_{2n}\\a_{31} & a_{32} & a_{33} & \cdots & a_{3n}\\\vdots & \vdots & \vdots & \ddots & \vdots\\a_{m1} & a_{m2} & a_{m3} & \cdots & a_{mn}\end{pmatrix}