\[{\theta_{2}}^{2}\left(0,q\right){\theta_{3}}^{2}\left(z,q\right)={\theta_{4}}^%
{2}\left(0,q\right){\theta_{1}}^{2}\left(z,q\right)+{\theta_{3}}^{2}\left(0,q%
\right){\theta_{2}}^{2}\left(z,q\right).\]