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