The quarter periods K and iK ′ are given by
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 and are periodic functions with periods and .
The quarter periods are essentially the elliptic integral of the first kind, by making the substitution . In this case, one writes instead of , understanding the difference between the two depends notationally on whether or is used. This notational difference has spawned a terminology to go with it:
- is called the parameter
- is called the complementary parameter
- is called the elliptic modulus
- is called the complementary elliptic modulus, where
- the modular angle, where
- the complementary modular angle. Note that
The elliptic modulus can be expressed in terms of the quarter periods as
where ns and dn Jacobian elliptic functions.
The nome is given by
The complementary nome is given by
The real quarter period can be expressed as a Lambert series involving the nome:
Additional expansions and relations can be found on the page for elliptic integrals.