The elliptic lambda function is a modular function defined on the upper halfplane by
(1)

where is the halfperiod ratio, is the nome
(2)

and are Jacobi theta functions.
The elliptic lambda function is essentially the same as the inverse nome, the difference being that elliptic lambda function is a function of the halfperiod ratio , while the inverse nome is a function of the nome , where is itself a function of .
It is implemented as the Wolfram Language function ModularLambda[tau].
The elliptic lambda function satisfies the functional equations
(3)
 
(4)

has the series expansion
(5)

(OEIS A115977), and has the series expansion
(6)

(OEIS A029845; Conway and Norton 1979; Borwein and Borwein 1987, p. 117).
gives the value of the elliptic modulus for which the complementary and normal complete elliptic integrals of the first kind are related by
(7)

i.e., the elliptic integral singular value for . It can be computed from
(8)

where
(9)

and is a Jacobi theta function. is related to by
(10)

For all rational , and are known as elliptic integral singular values, and can be expressed in terms of a finite number of gamma functions (Selberg and Chowla 1967). Values of for small include
(11)
 
(12)
 
(13)
 
(14)
 
(15)
 
(16)
 
(17)
 
(18)
 
(19)
 
(20)
 
(21)
 
(22)
 
(23)
 
(24)
 
(25)
 
(26)
 
(27)

where
(28)

The algebraic orders of these are given by 2, 2, 4, 2, 8, 4, 4, 4, 8, 4, 12, 4, 8, 8, 8, 4, ... (OEIS A084540).
Some additional exact values are given by
(29)
 
(30)
 
(31)
 
(32)
 
(33)
 
(34)

Exact values can also be found for rational , including
(35)
 
(36)
 
(37)
 
(38)
 
(39)
 
(40)
 
(41)
 
(42)
 
(43)
 
(44)

where is a polynomial root.
is related to the Ramanujan g and Gfunctions by
(45)
 
(46)
