\documentclass[12pt,leqno]{amsart}

\begin{document}

$$
F(z)=F_q(z;\lambda)=\sum_{n=0}^\infty\frac{z^n}{\prod_{j=1}^n(q^j-\lambda)},
\qquad |q|>1, \quad \lambda\notin q^{\mathbb {Z}_{>0}},
$$

\end{document}
