\[\sum_{n=-\infty}^{\infty}(-1)^{n}q^{n(3n-1)/2}z^{3n}(1+zq^{n})=\left(q,-z,-q/z%
;q\right)_{\infty}\left(qz^{2},q/{z^{2}};q^{2}\right)_{\infty}.\]