\[\operatorname{cn}\left(z,k\right)=\frac{\operatorname{cn}\left(z/(1+k_{1}),k_{%
1}\right)\operatorname{dn}\left(z/(1+k_{1}),k_{1}\right)}{1+k_{1}{%
\operatorname{sn}}^{2}\left(z/(1+k_{1}),k_{1}\right)},\]