Modular lambda function

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 ${\displaystyle \mathbb {C} /\langle 1,\tau \rangle }$, where the map is defined as the quotient by the [−1] involution.

Modular lambda function in the complex plane.

The q-expansion, where ${\displaystyle q=e^{\pi i\tau }}$ is the nome, is given by:

${\displaystyle \lambda (\tau )=16q-128q^{2}+704q^{3}-3072q^{4}+11488q^{5}-38400q^{6}+\dots }$. OEIS: A115977

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 ${\displaystyle \operatorname {SL} _{2}(\mathbb {Z} )}$, and it is in fact Klein's modular j-invariant.

A plot of x→ λ(ix)

Modular properties

The function ${\displaystyle \lambda (\tau )}$  is invariant under the group generated by^[1]

${\displaystyle \tau \mapsto \tau +2\ ;\ \tau \mapsto {\frac {\tau }{1-2\tau }}\ .}$ 

The generators of the modular group act by^[2]

${\displaystyle \tau \mapsto \tau +1\ :\ \lambda \mapsto {\frac {\lambda }{\lambda -1}}\,;}$ 

${\displaystyle \tau \mapsto -{\frac {1}{\tau }}\ :\ \lambda \mapsto 1-\lambda \ .}$ 

Consequently, the action of the modular group on ${\displaystyle \lambda (\tau )}$  is that of the anharmonic group, giving the six values of the cross-ratio:^[3]

${\displaystyle \left\lbrace {\lambda ,{\frac {1}{1-\lambda }},{\frac {\lambda -1}{\lambda }},{\frac {1}{\lambda }},{\frac {\lambda }{\lambda -1}},1-\lambda }\right\rbrace \ .}$ 

Relations to other functions

It is the square of the elliptic modulus,^[4] that is, ${\displaystyle \lambda (\tau )=k^{2}(\tau )}$ . In terms of the Dedekind eta function ${\displaystyle \eta (\tau )}$  and theta functions,^[4]

${\displaystyle \lambda (\tau )={\Bigg (}{\frac {{\sqrt {2}}\,\eta ({\tfrac {\tau }{2}})\eta ^{2}(2\tau )}{\eta ^{3}(\tau )}}{\Bigg )}^{8}={\frac {16}{\left({\frac {\eta (\tau /2)}{\eta (2\tau )}}\right)^{8}+16}}={\frac {\theta _{2}^{4}(\tau )}{\theta _{3}^{4}(\tau )}}}$ 

and,

${\displaystyle {\frac {1}{{\big (}\lambda (\tau ){\big )}^{1/4}}}-{\big (}\lambda (\tau ){\big )}^{1/4}={\frac {1}{2}}\left({\frac {\eta ({\tfrac {\tau }{4}})}{\eta (\tau )}}\right)^{4}=2\,{\frac {\theta _{4}^{2}({\tfrac {\tau }{2}})}{\theta _{2}^{2}({\tfrac {\tau }{2}})}}}$ 

where^[5]

${\displaystyle \theta _{2}(\tau )=\sum _{n=-\infty }^{\infty }e^{\pi i\tau (n+1/2)^{2}}}$ 

${\displaystyle \theta _{3}(\tau )=\sum _{n=-\infty }^{\infty }e^{\pi i\tau n^{2}}}$ 

${\displaystyle \theta _{4}(\tau )=\sum _{n=-\infty }^{\infty }(-1)^{n}e^{\pi i\tau n^{2}}}$ 

In terms of the half-periods of Weierstrass's elliptic functions, let ${\displaystyle [\omega _{1},\omega _{2}]}$  be a fundamental pair of periods with ${\displaystyle \tau ={\frac {\omega _{2}}{\omega _{1}}}}$ .

${\displaystyle e_{1}=\wp \left({\frac {\omega _{1}}{2}}\right),e_{2}=\wp \left({\frac {\omega _{2}}{2}}\right),e_{3}=\wp \left({\frac {\omega _{1}+\omega _{2}}{2}}\right)}$ 

we have^[4]

${\displaystyle \lambda ={\frac {e_{3}-e_{2}}{e_{1}-e_{2}}}\,.}$ 

Since the three half-period values are distinct, this shows that λ does not take the value 0 or 1.^[4]

The relation to the j-invariant is^[6][7]

${\displaystyle j(\tau )={\frac {256(1-\lambda (1-\lambda ))^{3}}{(\lambda (1-\lambda ))^{2}}}={\frac {256(1-\lambda +\lambda ^{2})^{3}}{\lambda ^{2}(1-\lambda )^{2}}}\ .}$ 

which is the j-invariant of the elliptic curve of Legendre form ${\displaystyle y^{2}=x(x-1)(x-\lambda )}$ 

Modular equations

The modular equation of degree ${\displaystyle p}$  (where ${\displaystyle p}$  is a prime number) is an algebraic equation in ${\displaystyle \alpha =\lambda (p\tau )}$  and ${\displaystyle \beta =\lambda (\tau )}$  (where ${\displaystyle \tau \in \mathbb {C} }$  such that ${\displaystyle \operatorname {Re} \tau =0}$  and ${\displaystyle \operatorname {Im} \tau >0}$ ). If ${\displaystyle u={\sqrt[{8}]{\alpha }}}$  and ${\displaystyle v={\sqrt[{8}]{\beta }}}$ , the modular equations of degrees ${\displaystyle 2,3,5,7}$  are, respectively,^[8]

${\displaystyle (1+u^{4})^{2}v^{8}-4u^{4}=0,}$ 

${\displaystyle u^{4}-v^{4}+2uv(1-u^{2}v^{2})=0,}$ 

${\displaystyle u^{6}-v^{6}+5u^{2}v^{2}(u^{2}-v^{2})+4uv(1-u^{4}v^{4})=0,}$ 

${\displaystyle (1-u^{8})(1-v^{8})-(1-uv)^{8}=0.}$ 

Lambda-star

 

λ*(x) in the complex plane.

Definition and computation of lambda-star

The function λ*(x)^[9] gives the value of the elliptic modulus ${\displaystyle k}$ , for which the complete elliptic integral of the first kind ${\displaystyle K(k)}$  and its complementary counterpart ${\displaystyle K\left({\sqrt {1-k^{2}}}\right)}$  are related by following expression:

${\displaystyle {\frac {K\left[{\sqrt {1-\lambda ^{*}(x)^{2}}}\right]}{K[\lambda ^{*}(x)]}}={\sqrt {x}}}$ 

The values of λ*(x) can be computed as follows:

${\displaystyle \lambda ^{*}(x)={\frac {\theta _{2}^{2}(i{\sqrt {x}})}{\theta _{3}^{2}(i{\sqrt {x}})}}}$ 

${\displaystyle \lambda ^{*}(x)=\left[\sum _{a=-\infty }^{\infty }\exp[-(a+1/2)^{2}\pi {\sqrt {x}}]\right]^{2}\left[\sum _{a=-\infty }^{\infty }\exp(-a^{2}\pi {\sqrt {x}})\right]^{-2}}$ 

${\displaystyle \lambda ^{*}(x)=\left[\sum _{a=-\infty }^{\infty }\operatorname {sech} [(a+1/2)\pi {\sqrt {x}}]\right]\left[\sum _{a=-\infty }^{\infty }\operatorname {sech} (a\pi {\sqrt {x}})\right]^{-1}}$ 

The functions λ* and λ are related to each other in this way:

${\displaystyle \lambda ^{*}(x)={\sqrt {\lambda (i{\sqrt {x}})}}}$ 

Properties of lambda-star

Every λ*-value of a positive rational number is a positive algebraic number:

${\displaystyle \lambda ^{*}(x\in \mathbb {Q} ^{+})\in \mathbb {A} ^{+}}$ 

Elliptic integrals of the first and second kind of these special λ*-values can be expressed in terms of the gamma function, as Selberg and Chowla proved in 1949.^[10][11]

Following expression is valid for all n ∈ ${\displaystyle \mathbb {N} }$ :

${\displaystyle {\sqrt {n}}=\sum _{a=1}^{n}\operatorname {dn} \left[{\frac {2a}{n}}K\left[\lambda ^{*}\left({\frac {1}{n}}\right)\right];\lambda ^{*}\left({\frac {1}{n}}\right)\right]}$ 

In this formula, dn is the Jacobi elliptic function delta amplitudinis.

By knowing one λ*-value, this formula can be used to compute related λ*-values:

${\displaystyle \lambda ^{*}(n^{2}x)=\lambda ^{*}(x)^{n}\prod _{a=1}^{n}\operatorname {sn} \left\{{\frac {2a-1}{n}}K[\lambda ^{*}(x)];\lambda ^{*}(x)\right\}^{2}}$ 

In that formula, sn is the Jacobi elliptic function sinus amplitudinis. That formula works for all natural numbers.

Further relations:

${\displaystyle \lambda ^{*}(x)^{2}+\lambda ^{*}(1/x)^{2}=1}$ 

${\displaystyle [\lambda ^{*}(x)+1][\lambda ^{*}(4/x)+1]=2}$ 

${\displaystyle \lambda ^{*}(4x)={\frac {1-{\sqrt {1-\lambda ^{*}(x)^{2}}}}{1+{\sqrt {1-\lambda ^{*}(x)^{2}}}}}=\tan \left\{{\frac {1}{2}}\arcsin[\lambda ^{*}(x)]\right\}^{2}}$ 

${\displaystyle \lambda ^{*}(x)-\lambda ^{*}(9x)=2[\lambda ^{*}(x)\lambda ^{*}(9x)]^{1/4}-2[\lambda ^{*}(x)\lambda ^{*}(9x)]^{3/4}}$ 

${\displaystyle \left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{1/2}-\left[{\frac {2\lambda ^{*}(25x)}{1-\lambda ^{*}(25x)^{2}}}\right]^{1/2}=2\left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{1/12}\left[{\frac {2\lambda ^{*}(25x)}{1-\lambda ^{*}(25x)^{2}}}\right]^{1/12}+2\left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{5/12}\left[{\frac {2\lambda ^{*}(25x)}{1-\lambda ^{*}(25x)^{2}}}\right]^{5/12}}$ 

${\displaystyle a^{8}+b^{8}-7a^{4}b^{4}=2{\sqrt {2}}ab+2{\sqrt {2}}a^{7}b^{7}\,\left(a=\left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{1/12}\right)\left(b=\left[{\frac {2\lambda ^{*}(49x)}{1-\lambda ^{*}(49x)^{2}}}\right]^{1/12}\right)}$ 

${\displaystyle a^{12}-c^{12}=2{\sqrt {2}}(ac+a^{3}c^{3})(1+3a^{2}c^{2}+a^{4}c^{4})(2+3a^{2}c^{2}+2a^{4}c^{4})\,\left(a=\left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{1/12}\right)\left(c=\left[{\frac {2\lambda ^{*}(121x)}{1-\lambda ^{*}(121x)^{2}}}\right]^{1/12}\right)}$ 

${\displaystyle (a^{2}-d^{2})(a^{4}+d^{4}-7a^{2}d^{2})[(a^{2}-d^{2})^{4}-a^{2}d^{2}(a^{2}+d^{2})^{2}]=8ad+8a^{13}d^{13}\,\left(a=\left[{\frac {2\lambda ^{*}(x)}{1-\lambda ^{*}(x)^{2}}}\right]^{1/12}\right)\left(d=\left[{\frac {2\lambda ^{*}(169x)}{1-\lambda ^{*}(169x)^{2}}}\right]^{1/12}\right)}$ 

Ramanujan's class invariants

Ramanujan's class invariants ${\displaystyle G_{n}}$  and ${\displaystyle g_{n}}$  are defined as^[12]

${\displaystyle G_{n}=2^{-1/4}e^{\pi {\sqrt {n}}/24}\prod _{k=0}^{\infty }\left(1+e^{-(2k+1)\pi {\sqrt {n}}}\right),}$ 

${\displaystyle g_{n}=2^{-1/4}e^{\pi {\sqrt {n}}/24}\prod _{k=0}^{\infty }\left(1-e^{-(2k+1)\pi {\sqrt {n}}}\right),}$ 

where ${\displaystyle n\in \mathbb {Q} ^{+}}$ .

These are the relations between lambda-star and Ramanujan's class invariants:

${\displaystyle G_{n}=\sin\{2\arcsin[\lambda ^{*}(n)]\}^{-1/12}=1{\Big /}\left[{\sqrt[{12}]{2\lambda ^{*}(n)}}{\sqrt[{24}]{1-\lambda ^{*}(n)^{2}}}\right]}$ 

${\displaystyle g_{n}=\tan\{2\arctan[\lambda ^{*}(n)]\}^{-1/12}={\sqrt[{12}]{[1-\lambda ^{*}(n)^{2}]/[2\lambda ^{*}(n)]}}}$ 

${\displaystyle \lambda ^{*}(n)=\tan \left\{{\frac {1}{2}}\arctan[g_{n}^{-12}]\right\}={\sqrt {g_{n}^{24}+1}}-g_{n}^{12}}$ 

Special Values

Lambda-star-values of integer numbers of 4n-3-type:

${\displaystyle \lambda ^{*}(1)={\frac {1}{\sqrt {2}}}}$ 

${\displaystyle \lambda ^{*}(5)=\sin \left[{\frac {1}{2}}\arcsin \left({\sqrt {5}}-2\right)\right]}$ 

${\displaystyle \lambda ^{*}(9)={\frac {1}{2}}({\sqrt {3}}-1)({\sqrt {2}}-{\sqrt[{4}]{3}})}$ 

${\displaystyle \lambda ^{*}(13)=\sin \left[{\frac {1}{2}}\arcsin(5{\sqrt {13}}-18)\right]}$ 

${\displaystyle \lambda ^{*}(17)=\sin \left\{{\frac {1}{2}}\arcsin \left[{\frac {1}{64}}\left(5+{\sqrt {17}}-{\sqrt {10{\sqrt {17}}+26}}\right)^{3}\right]\right\}}$ 

${\displaystyle \lambda ^{*}(21)=\sin \left\{{\frac {1}{2}}\arcsin[(8-3{\sqrt {7}})(2{\sqrt {7}}-3{\sqrt {3}})]\right\}}$ 

${\displaystyle \lambda ^{*}(25)={\frac {1}{\sqrt {2}}}({\sqrt {5}}-2)(3-2{\sqrt[{4}]{5}})}$ 

${\displaystyle \lambda ^{*}(33)=\sin \left\{{\frac {1}{2}}\arcsin[(10-3{\sqrt {11}})(2-{\sqrt {3}})^{3}]\right\}}$ 

${\displaystyle \lambda ^{*}(37)=\sin \left\{{\frac {1}{2}}\arcsin[({\sqrt {37}}-6)^{3}]\right\}}$ 

${\displaystyle \lambda ^{*}(45)=\sin \left\{{\frac {1}{2}}\arcsin[(4-{\sqrt {15}})^{2}({\sqrt {5}}-2)^{3}]\right\}}$ 

${\displaystyle \lambda ^{*}(49)={\frac {1}{4}}(8+3{\sqrt {7}})(5-{\sqrt {7}}-{\sqrt[{4}]{28}})\left({\sqrt {14}}-{\sqrt {2}}-{\sqrt[{8}]{28}}{\sqrt {5-{\sqrt {7}}}}\right)}$ 

${\displaystyle \lambda ^{*}(57)=\sin \left\{{\frac {1}{2}}\arcsin[(170-39{\sqrt {19}})(2-{\sqrt {3}})^{3}]\right\}}$ 

${\displaystyle \lambda ^{*}(73)=\sin \left\{{\frac {1}{2}}\arcsin \left[{\frac {1}{64}}\left(45+5{\sqrt {73}}-3{\sqrt {50{\sqrt {73}}+426}}\right)^{3}\right]\right\}}$ 

Lambda-star-values of integer numbers of 4n-2-type:

${\displaystyle \lambda ^{*}(2)={\sqrt {2}}-1}$ 

${\displaystyle \lambda ^{*}(6)=(2-{\sqrt {3}})({\sqrt {3}}-{\sqrt {2}})}$ 

${\displaystyle \lambda ^{*}(10)=({\sqrt {10}}-3)({\sqrt {2}}-1)^{2}}$ 

${\displaystyle \lambda ^{*}(14)=\tan \left\{{\frac {1}{2}}\arctan \left[{\frac {1}{8}}\left(2{\sqrt {2}}+1-{\sqrt {4{\sqrt {2}}+5}}\right)^{3}\right]\right\}}$ 

${\displaystyle \lambda ^{*}(18)=({\sqrt {2}}-1)^{3}(2-{\sqrt {3}})^{2}}$ 

${\displaystyle \lambda ^{*}(22)=(10-3{\sqrt {11}})(3{\sqrt {11}}-7{\sqrt {2}})}$ 

${\displaystyle \lambda ^{*}(30)=\tan \left\{{\frac {1}{2}}\arctan[({\sqrt {10}}-3)^{2}({\sqrt {5}}-2)^{2}]\right\}}$ 

${\displaystyle \lambda ^{*}(34)=\tan \left\{{\frac {1}{4}}\arcsin \left[{\frac {1}{9}}({\sqrt {17}}-4)^{2}\right]\right\}}$ 

${\displaystyle \lambda ^{*}(42)=\tan \left\{{\frac {1}{2}}\arctan[(2{\sqrt {7}}-3{\sqrt {3}})^{2}(2{\sqrt {2}}-{\sqrt {7}})^{2}]\right\}}$ 

${\displaystyle \lambda ^{*}(46)=\tan \left\{{\frac {1}{2}}\arctan \left[{\frac {1}{64}}\left(3+{\sqrt {2}}-{\sqrt {6{\sqrt {2}}+7}}\right)^{6}\right]\right\}}$ 

${\displaystyle \lambda ^{*}(58)=(13{\sqrt {58}}-99)({\sqrt {2}}-1)^{6}}$ 

${\displaystyle \lambda ^{*}(70)=\tan \left\{{\frac {1}{2}}\arctan[({\sqrt {5}}-2)^{4}({\sqrt {2}}-1)^{6}]\right\}}$ 

${\displaystyle \lambda ^{*}(78)=\tan \left\{{\frac {1}{2}}\arctan[(5{\sqrt {13}}-18)^{2}({\sqrt {26}}-5)^{2}]\right\}}$ 

${\displaystyle \lambda ^{*}(82)=\tan \left\{{\frac {1}{4}}\arcsin \left[{\frac {1}{4761}}(8{\sqrt {41}}-51)^{2}\right]\right\}}$ 

Lambda-star-values of integer numbers of 4n-1-type:

${\displaystyle \lambda ^{*}(3)={\frac {1}{2{\sqrt {2}}}}({\sqrt {3}}-1)}$ 

${\displaystyle \lambda ^{*}(7)={\frac {1}{4{\sqrt {2}}}}(3-{\sqrt {7}})}$ 

${\displaystyle \lambda ^{*}(11)={\frac {1}{8{\sqrt {2}}}}({\sqrt {11}}+3)\left({\frac {1}{3}}{\sqrt[{3}]{6{\sqrt {3}}+2{\sqrt {11}}}}-{\frac {1}{3}}{\sqrt[{3}]{6{\sqrt {3}}-2{\sqrt {11}}}}+{\frac {1}{3}}{\sqrt {11}}-1\right)^{4}}$ 

${\displaystyle \lambda ^{*}(15)={\frac {1}{8{\sqrt {2}}}}(3-{\sqrt {5}})({\sqrt {5}}-{\sqrt {3}})(2-{\sqrt {3}})}$ 

${\displaystyle \lambda ^{*}(19)={\frac {1}{8{\sqrt {2}}}}(3{\sqrt {19}}+13)\left[{\frac {1}{6}}({\sqrt {19}}-2+{\sqrt {3}}){\sqrt[{3}]{3{\sqrt {3}}-{\sqrt {19}}}}-{\frac {1}{6}}({\sqrt {19}}-2-{\sqrt {3}}){\sqrt[{3}]{3{\sqrt {3}}+{\sqrt {19}}}}-{\frac {1}{3}}(5-{\sqrt {19}})\right]^{4}}$ 

${\displaystyle \lambda ^{*}(23)={\frac {1}{16{\sqrt {2}}}}(5+{\sqrt {23}})\left[{\frac {1}{6}}({\sqrt {3}}+1){\sqrt[{3}]{100-12{\sqrt {69}}}}-{\frac {1}{6}}({\sqrt {3}}-1){\sqrt[{3}]{100+12{\sqrt {69}}}}+{\frac {2}{3}}\right]^{4}}$ 

${\displaystyle \lambda ^{*}(27)={\frac {1}{16{\sqrt {2}}}}({\sqrt {3}}-1)^{3}\left[{\frac {1}{3}}{\sqrt {3}}({\sqrt[{3}]{4}}-{\sqrt[{3}]{2}}+1)-{\sqrt[{3}]{2}}+1\right]^{4}}$ 

${\displaystyle \lambda ^{*}(39)=\sin \left\{{\frac {1}{2}}\arcsin \left[{\frac {1}{16}}\left(6-{\sqrt {13}}-3{\sqrt {6{\sqrt {13}}-21}}\right)\right]\right\}}$ 

${\displaystyle \lambda ^{*}(55)=\sin \left\{{\frac {1}{2}}\arcsin \left[{\frac {1}{512}}\left(3{\sqrt {5}}-3-{\sqrt {6{\sqrt {5}}-2}}\right)^{3}\right]\right\}}$ 

Lambda-star-values of integer numbers of 4n-type:

${\displaystyle \lambda ^{*}(4)=({\sqrt {2}}-1)^{2}}$ 

${\displaystyle \lambda ^{*}(8)=\left({\sqrt {2}}+1-{\sqrt {2{\sqrt {2}}+2}}\right)^{2}}$ 

${\displaystyle \lambda ^{*}(12)=({\sqrt {3}}-{\sqrt {2}})^{2}({\sqrt {2}}-1)^{2}}$ 

${\displaystyle \lambda ^{*}(16)=({\sqrt {2}}+1)^{2}({\sqrt[{4}]{2}}-1)^{4}}$ 

${\displaystyle \lambda ^{*}(20)=\tan \left[{\frac {1}{4}}\arcsin({\sqrt {5}}-2)\right]^{2}}$ 

${\displaystyle \lambda ^{*}(24)=\tan \left\{{\frac {1}{2}}\arcsin[(2-{\sqrt {3}})({\sqrt {3}}-{\sqrt {2}})]\right\}^{2}}$ 

${\displaystyle \lambda ^{*}(28)=(2{\sqrt {2}}-{\sqrt {7}})^{2}({\sqrt {2}}-1)^{4}}$ 

${\displaystyle \lambda ^{*}(32)=\tan \left\{{\frac {1}{2}}\arcsin \left[\left({\sqrt {2}}+1-{\sqrt {2{\sqrt {2}}+2}}\right)^{2}\right]\right\}^{2}}$ 

Lambda-star-values of rational fractions:

${\displaystyle \lambda ^{*}\left({\frac {1}{2}}\right)={\sqrt {2{\sqrt {2}}-2}}}$ 

${\displaystyle \lambda ^{*}\left({\frac {1}{3}}\right)={\frac {1}{2{\sqrt {2}}}}({\sqrt {3}}+1)}$ 

${\displaystyle \lambda ^{*}\left({\frac {2}{3}}\right)=(2-{\sqrt {3}})({\sqrt {3}}+{\sqrt {2}})}$ 

${\displaystyle \lambda ^{*}\left({\frac {1}{4}}\right)=2{\sqrt[{4}]{2}}({\sqrt {2}}-1)}$ 

${\displaystyle \lambda ^{*}\left({\frac {3}{4}}\right)={\sqrt[{4}]{8}}({\sqrt {3}}-{\sqrt {2}})({\sqrt {2}}+1){\sqrt {({\sqrt {3}}-1)^{3}}}}$ 

${\displaystyle \lambda ^{*}\left({\frac {1}{5}}\right)={\frac {1}{2{\sqrt {2}}}}\left({\sqrt {2{\sqrt {5}}-2}}+{\sqrt {5}}-1\right)}$ 

${\displaystyle \lambda ^{*}\left({\frac {2}{5}}\right)=({\sqrt {10}}-3)({\sqrt {2}}+1)^{2}}$ 

${\displaystyle \lambda ^{*}\left({\frac {3}{5}}\right)={\frac {1}{8{\sqrt {2}}}}(3+{\sqrt {5}})({\sqrt {5}}-{\sqrt {3}})(2+{\sqrt {3}})}$ 

${\displaystyle \lambda ^{*}\left({\frac {4}{5}}\right)=\tan \left[{\frac {\pi }{4}}-{\frac {1}{4}}\arcsin({\sqrt {5}}-2)\right]^{2}}$ 

Other appearances

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.^[13] 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.^[14]

Moonshine

The function ${\displaystyle \tau \mapsto {\frac {16}{\lambda (2\tau )}}-8}$  is the normalized Hauptmodul for the group ${\displaystyle \Gamma _{0}(4)}$ , and its q-expansion ${\displaystyle q^{-1}+20q-62q^{3}+\dots }$ , OEIS: A007248 where ${\displaystyle q=e^{2\pi i\tau }}$ , is the graded character of any element in conjugacy class 4C of the monster group acting on the monster vertex algebra.

Footnotes

1. Chandrasekharan (1985) p.115
2. Chandrasekharan (1985) p.109
3. Chandrasekharan (1985) p.110
4. Chandrasekharan (1985) p.108
5. Chandrasekharan (1985) p.63
6. Chandrasekharan (1985) p.117
7. Rankin (1977) pp.226–228
8. Borwein, Jonathan M.; Borwein, Peter B. (1987). Pi and the AGM: A Study in Analytic Number Theory and Computational Complexity (First ed.). Wiley-Interscience. ISBN 0-471-83138-7. p. 103–109, 134
9. Borwein, Jonathan M.; Borwein, Peter B. (1987). Pi and the AGM: A Study in Analytic Number Theory and Computational Complexity (First ed.). Wiley-Interscience. ISBN 0-471-83138-7. p. 152
10. Chowla, S.; Selberg, A. "On Epstein's Zeta Function (I)". Semantic Scholar. p. 373.
11. Chowla, S.; Selberg, A. "On Epstein's Zeta-Function". EuDML. p. 86–110.
12. Zhang, Liang-Cheng "Ramanujan's class invariants, Kronecker's limit formula and modular equations (III)"
13. Chandrasekharan (1985) p.121
14. Chandrasekharan (1985) p.118

References

• Abramowitz, Milton; Stegun, Irene A., eds. (1972), Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, New York: Dover Publications, ISBN 978-0-486-61272-0, Zbl 0543.33001
• Chandrasekharan, K. (1985), Elliptic Functions, Grundlehren der mathematischen Wissenschaften, vol. 281, Springer-Verlag, pp. 108–121, ISBN 3-540-15295-4, Zbl 0575.33001
• Conway, John Horton; Norton, Simon (1979), "Monstrous moonshine", Bulletin of the London Mathematical Society, 11 (3): 308–339, doi:10.1112/blms/11.3.308, MR 0554399, Zbl 0424.20010
• Rankin, Robert A. (1977), Modular Forms and Functions, Cambridge University Press, ISBN 0-521-21212-X, Zbl 0376.10020
• Reinhardt, W. P.; Walker, P. L. (2010), "Elliptic Modular Function", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248.

• Borwein, J. M. and Borwein, P. B. Pi & the AGM: A Study in Analytic Number Theory and Computational Complexity. New York: Wiley, pp. 139 and 298, 1987.

• Conway, J. H. and Norton, S. P. "Monstrous Moonshine." Bull. London Math. Soc. 11, 308-339, 1979.

• Selberg, A. and Chowla, S. "On Epstein's Zeta-Function." J. reine angew. Math. 227, 86-110, 1967.

External links

• Modular lambda function at Fungrim