\[\mathcal{E}\left(x,k\right)=\frac{1}{{\theta_{3}}^{2}\left(0,q\right)\theta_{4%
}\left(\xi,q\right)}\frac{\mathrm{d}}{\mathrm{d}\xi}\theta_{4}\left(\xi,q%
\right)+\frac{E\left(k\right)}{K\left(k\right)}x,\]