# Quarter period

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 ${\rm {sn}}u\,$ and ${\rm {cn}}u\,$ are periodic functions with periods $4K\,$ and $4{\rm {i}}K'\,$ .

## 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

$k={\textrm {ns}}(K+{\rm {i}}K')\,\!$

and

$k'={\textrm {dn}}K\,$

where ns and dn 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.