In mathematics, the modular lambda function λ(τ) 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. The function is invariant under the group generated by The generators of the modular group act by Consequently, the action of the modular group on is that of the anharmonic group, giving the six values of the cross-ratio: It is the square of the elliptic modulus, that is, . In terms of the Dedekind eta function and theta functions, and, where In terms of the half-periods of Weierstrass's elliptic functions, let be a fundamental pair of periods with . we have Since the three half-period values are distinct, this shows that does not take the value 0 or 1. The relation to the j-invariant is 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 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, 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 . The algebraic values of are also given by where is the lemniscate sine and is the lemniscate constant.

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Nome (mathematics)
In mathematics, specifically the theory of elliptic functions, the nome is a special function that belongs to the non-elementary functions. This function is of great importance in the description of the elliptic functions, especially in the description of the modular identity of the Jacobi theta function, the Hermite elliptic transcendents and the Weber modular functions, that are used for solving equations of higher degrees. The nome function is given by where and are the quarter periods, and and are the fundamental pair of periods, and is the half-period ratio.
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 and 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.
Fundamental pair of periods
In mathematics, a fundamental pair of periods is an ordered pair of complex numbers that defines a lattice in the complex plane. This type of lattice is the underlying object with which elliptic functions and modular forms are defined. A fundamental pair of periods is a pair of complex numbers such that their ratio is not real. If considered as vectors in , the two are not collinear. The lattice generated by and is This lattice is also sometimes denoted as to make clear that it depends on and It is also sometimes denoted by or or simply by The two generators and are called the lattice basis.
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