Jacobi triple product (source code)

= Jacobi triple product
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For $|q|<1$ and $z\ne0$, the Jacobi triple product is
$$
\sum_{n\in\mathbb Z}z^nq^{n^2}
=\prod_{m\geq1}(1-q^{2m})(1+zq^{2m-1})(1+z^{-1}q^{2m-1}).
$$