% Fichier ex4.4.7-2.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$$\int_{-\infty}^{\infty} 
    e^{-x^{2}} \, dx$$

$$\int\limits_{-\infty}^{\infty} 
    e^{-x^{2}} \, dx$$
\end{document}
