% Fichier exo4.10.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$$
\left( \frac{\partial^2}
            {\partial x^2}
      +\frac{\partial^2}
            {\partial y^2}
\right) |\varphi(x+iy)|^2=0
$$
\end{document}
