Modular lambda function
Symmetric holomorphic function From Wikipedia, the free encyclopedia
In mathematics, the modular lambda function λ(τ)[note 1] is a highly symmetric holomorphic function on the complex upper half-plane. It is invariant under the fractional linear action of the congruence group Γ(2), and generates the function field of the corresponding quotient, i.e., it is a Hauptmodul for the modular curve X(2). Over any point τ, its value can be described as a cross ratio of the branch points of a ramified double cover of the projective line by the elliptic curve , where the map is defined as the quotient by the [−1] involution.

The q-expansion, where is the nome, is given by:
By symmetrizing the lambda function under the canonical action of the symmetric group S3 on X(2), and then normalizing suitably, one obtains a function on the upper half-plane that is invariant under the full modular group , and it is in fact Klein's modular j-invariant.

Modular properties
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Perspective
The function is invariant under the group generated by[1]
The generators of the modular group act by[2]
Consequently, the action of the modular group on is that of the anharmonic group, giving the six values of the cross-ratio:[3]
Relations to other functions
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Perspective
It is the square of the elliptic modulus,[4] that is, . In terms of the Dedekind eta function and theta functions,[4]
and,
where[5]
In terms of the half-periods of Weierstrass's elliptic functions, let be a fundamental pair of periods with .
we have[4]
Since the three half-period values are distinct, this shows that does not take the value 0 or 1.[4]
The relation to the j-invariant is[6][7]
which is the j-invariant of the elliptic curve of Legendre form
Given , let
where is the complete elliptic integral of the first kind with parameter . Then
Modular equations
The modular equation of degree (where is a prime number) is an algebraic equation in and . If and , the modular equations of degrees are, respectively,[8]
The quantity (and hence ) can be thought of as a holomorphic function on the upper half-plane :
Since , the modular equations can be used to give algebraic values of for any prime .[note 2] The algebraic values of are also given by[9][note 3]
where is the lemniscate sine and is the lemniscate constant.
Lambda-star
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Perspective
Definition and computation of lambda-star
The function [10] (where ) gives the value of the elliptic modulus , for which the complete elliptic integral of the first kind and its complementary counterpart are related by following expression:
The values of can be computed as follows:
The functions and are related to each other in this way:
Properties of lambda-star
Every value of a positive rational number is a positive algebraic number:
and (the complete elliptic integral of the second kind) can be expressed in closed form in terms of the gamma function for any , as Selberg and Chowla proved in 1949.[11][12]
The following expression is valid for all :
where is the Jacobi elliptic function delta amplitudinis with modulus .
By knowing one value, this formula can be used to compute related values:[9]
where and is the Jacobi elliptic function sinus amplitudinis with modulus .
Further relations:
Special values |
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Lambda-star values of integer numbers of 4n-3-type: Lambda-star values of integer numbers of 4n-2-type: Lambda-star values of integer numbers of 4n-1-type: Lambda-star values of integer numbers of 4n-type: Lambda-star values of rational fractions: |
Ramanujan's class invariants
Ramanujan's class invariants and are defined as[13]
where . For such , the class invariants are algebraic numbers. For example
Identities with the class invariants include[14]
The class invariants are very closely related to the Weber modular functions and . These are the relations between lambda-star and the class invariants:
Other appearances
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Perspective
Little Picard theorem
The lambda function is used in the original proof of the Little Picard theorem, that an entire non-constant function on the complex plane cannot omit more than one value. This theorem was proved by Picard in 1879.[15] Suppose if possible that f is entire and does not take the values 0 and 1. Since λ is holomorphic, it has a local holomorphic inverse ω defined away from 0,1,∞. Consider the function z → ω(f(z)). By the Monodromy theorem this is holomorphic and maps the complex plane C to the upper half plane. From this it is easy to construct a holomorphic function from C to the unit disc, which by Liouville's theorem must be constant.[16]
Moonshine
The function is the normalized Hauptmodul for the group , and its q-expansion , OEIS: A007248 where , is the graded character of any element in conjugacy class 4C of the monster group acting on the monster vertex algebra.
Footnotes
References
External links
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