\[\mathcal{E}\left(x,k\right)={k^{\prime}}^{2}\int_{0}^{x}{\operatorname{nd}}^{2%
}\left(t,k\right)\,\mathrm{d}t+k^{2}\operatorname{sn}\left(x,k\right)%
\operatorname{cd}\left(x,k\right).\]