Q-theta function
From specialfunctionswiki
For $0 \leq |q| < 1$, $$\theta(z;q)=\prod_{k=0}^{\infty} (1-q^kz) \left(1-\dfrac{q^{k+1}}{z} \right)=(z;q)_{\infty}\left(\frac{q}{z};q \right)_{\infty},$$ where $(a,b)_{\infty}$ is the $q$-Pochhammer symbol.
