The modular equation of degree gives an algebraic connection of the form
(1)
between the transcendental complete elliptic integrals of the first kind with moduli and . When and satisfy a modular equation, a relationship of the form
(2)
exists, and is called the multiplier. In general, if is an odd prime, then the modular equation is given by
(3)
where
is a elliptic lambda function, and
(6)
(Borwein and Borwein 1987, p. 126), where is the half-period ratio. An elliptic integral identity gives
(7)
so the modular equation of degree 2 is
(8)
which can be written as
(9)
A few low order modular equations written in terms of and are
In terms of and ,
where
(17)
and
(18)
Here, are Jacobi theta functions.
A modular equation of degree for can be obtained by iterating the equation for . Modular equations for prime from 3 to 23 are given in Borwein and Borwein (1987).
Quadratic modular identities include
(19)
Cubic identities include
(20)
(21)
(22)
A seventh-order identity is
(23)
From Ramanujan (1913-1914),
(24)
(25)
When and satisfy a modular equation, a relationship of the form
(26)
exists, and is called the multiplier. The multiplier of degree can be given by
(27)
where is a Jacobi theta function and is a complete elliptic integral of the first kind.
The first few multipliers in terms of and are
In terms of the and defined for modular equations,
See alsoModular Form,
Modular Function,
Schläfli's Modular Form Explore with Wolfram|Alpha ReferencesBorwein, J. M. and Borwein, P. B. Pi & the AGM: A Study in Analytic Number Theory and Computational Complexity. New York: Wiley, pp. 127-132, 1987.Hanna, M. "The Modular Equations." Proc. London Math. Soc. 28, 46-52, 1928.Ramanujan, S. "Modular Equations and Approximations to ." Quart. J. Pure. Appl. Math. 45, 350-372, 1913-1914. Referenced on Wolfram|AlphaModular Equation Cite this as:Weisstein, Eric W. "Modular Equation." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/ModularEquation.html
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