# §22.17(i) Real or Purely Imaginary Moduli

Jacobian elliptic functions with real moduli in the intervals $(-\infty,0)$ and $(1,\infty)$, or with purely imaginary moduli are related to functions with moduli in the interval $[0,1]$ by the following formulas.

First

 22.17.1 $\mathop{\mathrm{pq}\/}\nolimits\left(z,k\right)=\mathop{\mathrm{pq}\/}% \nolimits\left(z,-k\right),$

for all twelve functions.

Secondly,

 22.17.2 $\displaystyle\mathop{\mathrm{sn}\/}\nolimits\left(z,1/k\right)$ $\displaystyle=k\mathop{\mathrm{sn}\/}\nolimits\left(z/k,k\right),$ Symbols: $\mathop{\mathrm{sn}\/}\nolimits\left(z,k\right)$: Jacobian elliptic function, $z$: complex and $k$: modulus A&S Ref: 16.11.2 Permalink: http://dlmf.nist.gov/22.17.E2 Encodings: TeX, pMML, png 22.17.3 $\displaystyle\mathop{\mathrm{cn}\/}\nolimits\left(z,1/k\right)$ $\displaystyle=\mathop{\mathrm{dn}\/}\nolimits\left(z/k,k\right),$ 22.17.4 $\displaystyle\mathop{\mathrm{dn}\/}\nolimits\left(z,1/k\right)$ $\displaystyle=\mathop{\mathrm{cn}\/}\nolimits\left(z/k,k\right).$

Thirdly, with

 22.17.5 $\displaystyle k_{1}$ $\displaystyle=\frac{k}{\sqrt{1+k^{2}}},$ $\displaystyle k_{1}k^{\prime}_{1}$ $\displaystyle=\frac{k}{1+k^{2}},$ Symbols: $k$: modulus and $k^{\prime}$: complementary modulus A&S Ref: 16.10.1 (modified) Referenced by: §22.17(i) Permalink: http://dlmf.nist.gov/22.17.E5 Encodings: TeX, TeX, pMML, pMML, png, png
 22.17.6 $\displaystyle\mathop{\mathrm{sn}\/}\nolimits\left(z,ik\right)$ $\displaystyle=k^{\prime}_{1}\mathop{\mathrm{sd}\/}\nolimits\left(z/k^{\prime}_% {1},k_{1}\right),$ 22.17.7 $\displaystyle\mathop{\mathrm{cn}\/}\nolimits\left(z,ik\right)$ $\displaystyle=\mathop{\mathrm{cd}\/}\nolimits\left(z/k^{\prime}_{1},k_{1}% \right),$ 22.17.8 $\displaystyle\mathop{\mathrm{dn}\/}\nolimits\left(z,ik\right)$ $\displaystyle=\mathop{\mathrm{nd}\/}\nolimits\left(z/k^{\prime}_{1},k_{1}% \right).$

In terms of the coefficients of the power series of §22.10(i), the above equations are polynomial identities in $k$. In (22.17.5) either value of the square root can be chosen.

# §22.17(ii) Complex Moduli

When $z$ is fixed each of the twelve Jacobian elliptic functions is a meromorphic function of $k^{2}$. For illustrations see Figures 22.3.2522.3.29. In consequence, the formulas in this chapter remain valid when $k$ is complex. In particular, the Landen transformations in §§22.7(i) and 22.7(ii) are valid for all complex values of $k$, irrespective of which values of $\sqrt{k}$ and $k^{\prime}=\sqrt{1-k^{2}}$ are chosen—as long as they are used consistently. For proofs of these results and further information see Walker (2003).