\[\theta_{3}\left(z,q\right)\theta_{3}\left(w,q\right)=\theta_{3}\left(z+w,q^{2}%
\right)\theta_{3}\left(z-w,q^{2}\right)+\theta_{2}\left(z+w,q^{2}\right)\theta%
_{2}\left(z-w,q^{2}\right).\]