% Fichier ex4.4.8-1.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$$ \mathcal{D} =
   \begin{array}{|ccc|}
   1 & 2 & 3 \\
   4 & 5 & 6 \\
   7 & 8 & 9\\
   \end{array}$$
\end{document}
