% Fichier ex4.4.12-2.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
\boldmath
$$
\prod\limits_{k\ge 0}
\frac{1}{1-q^kz}
$$
\unboldmath
$$
\mathbf{\prod\limits_{k\ge 0}
\frac{1}{1-q^kz}}
$$
\end{document}
