
\documentclass{article}
  \usepackage{lmodern}
  \usepackage{graphics}
  \pagestyle{empty}

\begin{document}
\[
\begin{array}{ccc}
   x_{11} & \cdots & x_{1p} \\
   \vdots & \ddots & \vdots \\
   x_{n1} & \cdots & x_{np}
\end{array}
\]
\end{document}
