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## Summary

In mathematics, the quarter periods K(m) and iK ′(m) are special functions that appear in the theory of elliptic functions.

The quarter periods K and iK ′ are given by

$K(m)=\int _{0}^{\frac {\pi }{2}}{\frac {d\theta }{\sqrt {1-m\sin ^{2}\theta }}}$ and

${\rm {i}}K'(m)={\rm {i}}K(1-m).\,$ When m is a real number, 0 < m < 1, then both K and K ′ are real numbers. By convention, K is called the real quarter period and iK ′ is called the imaginary quarter period. Any one of the numbers m, K, K ′, or K ′/K uniquely determines the others.

These functions appear in the theory of Jacobian elliptic functions; they are called quarter periods because the elliptic functions $\operatorname {sn} u$ and $\operatorname {cn} u$ are periodic functions with periods $4K$ and $4{\rm {i}}K'.$ However, the $\operatorname {sn}$ function is also periodic with a smaller period (in terms of the absolute value) than $4\mathrm {i} K'$ , namely $2\mathrm {i} K'$ .

## Notation

The quarter periods are essentially the elliptic integral of the first kind, by making the substitution $k^{2}=m$ . In this case, one writes $K(k)\,$  instead of $K(m)$ , understanding the difference between the two depends notationally on whether $k$  or $m$  is used. This notational difference has spawned a terminology to go with it:

• $m$  is called the parameter
• $m_{1}=1-m$  is called the complementary parameter
• $k$  is called the elliptic modulus
• $k'$  is called the complementary elliptic modulus, where ${k'}^{2}=m_{1}$
• $\alpha$  the modular angle, where $k=\sin \alpha ,$
• ${\frac {\pi }{2}}-\alpha$  the complementary modular angle. Note that
$m_{1}=\sin ^{2}\left({\frac {\pi }{2}}-\alpha \right)=\cos ^{2}\alpha .$

The elliptic modulus can be expressed in terms of the quarter periods as

$k=\operatorname {ns} (K+{\rm {i}}K')$

and

$k'=\operatorname {dn} K$

where $\operatorname {ns}$  and $\operatorname {dn}$  are Jacobian elliptic functions.

The nome $q\,$  is given by

$q=e^{-{\frac {\pi K'}{K}}}.$

The complementary nome is given by

$q_{1}=e^{-{\frac {\pi K}{K'}}}.$

The real quarter period can be expressed as a Lambert series involving the nome:

$K={\frac {\pi }{2}}+2\pi \sum _{n=1}^{\infty }{\frac {q^{n}}{1+q^{2n}}}.$

Additional expansions and relations can be found on the page for elliptic integrals.