\documentclass[a4paper,10pt]{article}
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 \usepackage{amsmath} 

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\begin{document}
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\section{A matrix with row and column descriptions}

           $T_a = \bordermatrix{~ & s'_1 & \dots & s'_n \cr
                  s_1 & t_{1,1} & \dots & t_{1,n} \cr
                  \dots & \dots & \dots & \dots \cr
                  s_n & t_{n,1} & \dots & t_{n,n} \cr}$


\section{A standard matrix}

pmatrix requires amsmath 

$A_{m,n} =
 \begin{pmatrix}
  a_{1,1} & a_{1,2} & \cdots & a_{1,n} \\
  a_{2,1} & a_{2,2} & \cdots & a_{2,n} \\
  \vdots  & \vdots  & \ddots & \vdots  \\
  a_{m,1} & a_{m,2} & \cdots & a_{m,n}
 \end{pmatrix}$

$A_{m,n} =
 \begin{bmatrix}
  a_{1,1} & a_{1,2} & \cdots & a_{1,n} \\
  a_{2,1} & a_{2,2} & \cdots & a_{2,n} \\
  \vdots  & \vdots  & \ddots & \vdots  \\
  a_{m,1} & a_{m,2} & \cdots & a_{m,n}
 \end{bmatrix}$


\end{document}
