# Glaisher–Kinkelin constant

In mathematics, the Glaisher–Kinkelin constant or Glaisher's constant, typically denoted A, is a mathematical constant, related to the K-function and the Barnes G-function. The constant appears in a number of sums and integrals, especially those involving gamma functions and zeta functions. It is named after mathematicians James Whitbread Lee Glaisher and Hermann Kinkelin.

Its approximate value is:

$A\approx 1.2824271291\dots$ (sequence A074962 in the OEIS).

The Glaisher–Kinkelin constant $A$ can be given by the limit:

$A=\lim _{n\rightarrow \infty }{\frac {K(n+1)}{n^{n^{2}/2+n/2+1/12}\,e^{-n^{2}/4}}}$ where $K(n)=\prod _{k=1}^{n-1}k^{k}$ is the K-function. This formula displays a similarity between A and π which is perhaps best illustrated by noting Stirling's formula:

${\sqrt {2\pi }}=\lim _{n\to \infty }{\frac {n!}{n^{n+1/2}\,e^{-n}}}$ which shows that just as π is obtained from approximation of the function $\prod _{k=1}^{n}k$ , A can also be obtained from a similar approximation to the function $\prod _{k=1}^{n}k^{k}$ .
An equivalent definition for A involving the Barnes G-function, given by $G(n)=\prod _{k=1}^{n-2}k!={\frac {\left[\Gamma (n)\right]^{n-1}}{K(n)}}$ where $\Gamma (n)$ is the gamma function is:

$A=\lim _{n\rightarrow \infty }{\frac {(2\pi )^{n/2}n^{n^{2}/2-1/12}e^{-3n^{2}/4+1/12}}{G(n+1)}}$ .

The Glaisher–Kinkelin constant also appears in evaluations of the derivatives of the Riemann zeta function, such as:

$\zeta ^{\prime }(-1)={\frac {1}{12}}-\ln A$ $\sum _{k=2}^{\infty }{\frac {\ln k}{k^{2}}}=-\zeta ^{\prime }(2)={\frac {\pi ^{2}}{6}}\left[12\ln A-\gamma -\ln(2\pi )\right]$ where $\gamma$ is the Euler–Mascheroni constant. The latter formula leads directly to the following product found by Glaisher:

$\prod _{k=1}^{\infty }k^{1/k^{2}}=\left({\frac {A^{12}}{2\pi e^{\gamma }}}\right)^{\pi ^{2}/6}$ An alternative product formula, defined over the prime numbers, reads 

$\prod _{k=1}^{\infty }p_{k}^{1/(p_{k}^{2}-1)}={\frac {A^{12}}{2\pi e^{\gamma }}},$ where $p_{k}$ denotes the $k$ th prime number.

The following are some integrals that involve this constant:

$\int _{0}^{1/2}\ln \Gamma (x)\,dx={\frac {3}{2}}\ln A+{\frac {5}{24}}\ln 2+{\frac {1}{4}}\ln \pi$ $\int _{0}^{\infty }{\frac {x\ln x}{e^{2\pi x}-1}}\,dx={\frac {1}{2}}\zeta ^{\prime }(-1)={\frac {1}{24}}-{\frac {1}{2}}\ln A$ A series representation for this constant follows from a series for the Riemann zeta function given by Helmut Hasse.

$\ln A={\frac {1}{8}}-{\frac {1}{2}}\sum _{n=0}^{\infty }{\frac {1}{n+1}}\sum _{k=0}^{n}(-1)^{k}{\binom {n}{k}}(k+1)^{2}\ln(k+1)$ 