Lemniscate constant

Not to be confused with Gaussian gravitational constant.

In mathematics, Gauss's constant, denoted by G, is defined as the reciprocal of the arithmetic–geometric mean of 1 and the square root of 2:

$G={\frac {1}{\operatorname {agm} \left(1,{\sqrt {2}}\right)}}=0.8346268\dots .$

The constant is named after Carl Friedrich Gauss, who in 1799 discovered that

$G={\frac {2}{\pi }}\int _{0}^{1}{\frac {dx}{\sqrt {1-x^{4}}}}$

so that

$G={\frac {1}{2\pi }}\mathrm {B} \left({\tfrac {1}{4}},{\tfrac {1}{2}}\right)$

where Β denotes the beta function.

Relations to other constants

Gauss's constant may be used to express the gamma function at argument ⁠1/4⁠:

$\Gamma \left({\tfrac {1}{4}}\right)={\sqrt {2G{\sqrt {2\pi ^{3}}}}}$

Alternatively,

$G={\frac {\left[\Gamma \left({\tfrac {1}{4}}\right)\right]^{2}}{2{\sqrt {2\pi ^{3}}}}}$

and since π and Γ(⁠1/4⁠) are algebraically independent, Gauss's constant is transcendental.

Lemniscate constants

Gauss's constant may be used in the definition of the lemniscate constants, the first of which is:

$L_{1}=\pi G$

and the second constant:

$L_{2}={\frac {1}{2G}}$

which arise in finding the arc length of a lemniscate. Both constants were proven to be transcendental.

Other formulas

A formula for G in terms of Jacobi theta functions is given by

$G=\vartheta _{01}^{2}\left(e^{-\pi }\right)$

as well as the rapidly converging series

$G={\sqrt[{4}]{32}}e^{-{\frac {\pi }{3}}}\left(\sum _{n=-\infty }^{\infty }(-1)^{n}e^{-2n\pi (3n+1)}\right)^{2}.$

The constant is also given by the infinite product

$G=\prod _{m=1}^{\infty }\tanh ^{2}\left({\frac {\pi m}{2}}\right).$

It appears in the evaluation of the integrals

${\frac {1}{G}}=\int _{0}^{\frac {\pi }{2}}{\sqrt {\sin(x)}}\,dx=\int _{0}^{\frac {\pi }{2}}{\sqrt {\cos(x)}}\,dx$

$G=\int _{0}^{\infty }{\frac {dx}{\sqrt {\cosh(\pi x)}}}$

Gauss' constant as a continued fraction is [0, 1, 5, 21, 3, 4, 14, ...]. (sequence A053002 in the OEIS)

See also

• Lemniscatic elliptic function

References

• Weisstein, Eric W. "Gauss's Constant". MathWorld.
• Sequences A014549 and A053002 in OEIS