Jacobi triple product
In mathematics, the Jacobi triple product is the mathematical identity:
\prod_{m=1}^\infty
\left
\left
\left
= \sum_{n=-\infty}^\infty x^{n^2} y^{2n},
for complex numbers x and y, with |x| < 1 and y ≠ 0.
It was introduced by Jacobi in his work... Wikipedia