\[\frac{{\mathrm{d}}^{2}}{{\mathrm{d}z}^{2}}\operatorname{ds}\left(z,k\right)=(k%
^{2}-{k^{\prime}}^{2})\operatorname{ds}\left(z,k\right)+2{\operatorname{ds}}^{%
3}\left(z,k\right),\]