# Multiple gamma function

In mathematics, the multiple gamma function $\Gamma _{N}$ is a generalization of the Euler gamma function and the Barnes G-function. The double gamma function was studied by Barnes (1901). At the end of this paper he mentioned the existence of multiple gamma functions generalizing it, and studied these further in Barnes (1904).

Double gamma functions $\Gamma _{2}$ are closely related to the q-gamma function, and triple gamma functions $\Gamma _{3}$ are related to the elliptic gamma function.

## Definition

For $\Re a_{i}>0$ , let

$\Gamma _{N}(w\mid a_{1},\ldots ,a_{N})=\exp \left(\left.{\frac {\partial }{\partial s}}\zeta _{N}(s,w\mid a_{1},\ldots ,a_{N})\right|_{s=0}\right)\ ,$

where $\zeta _{N}$  is the Barnes zeta function. (This differs by a constant from Barnes's original definition.)

## Properties

Considered as a meromorphic function of $w$ , $\Gamma _{N}(w\mid a_{1},\ldots ,a_{N})$  has no zeros. It has poles at $w=-\sum _{i=1}^{N}n_{i}a_{i}$ for non-negative integers $n_{i}$ . These poles are simple unless some of them coincide. Up to multiplication by the exponential of a polynomial, $\Gamma _{N}(w\mid a_{1},\ldots ,a_{N})$  is the unique meromorphic function of finite order with these zeros and poles.

• $\Gamma _{0}(w\mid )={\frac {1}{w}}\ ,$
• $\Gamma _{1}(w\mid a)={\frac {a^{a^{-1}w-{\frac {1}{2}}}}{\sqrt {2\pi }}}\Gamma \left(a^{-1}w\right)\ ,$
• $\Gamma _{N}(w\mid a_{1},\ldots ,a_{N})=\Gamma _{N-1}(w\mid a_{1},\ldots ,a_{N-1})\Gamma _{N}(w+a_{N}\mid a_{1},\ldots ,a_{N})\ .$

## Infinite product representation

The multiple gamma function has an infinite product representation that makes it manifest that it is meromorphic, and that also makes the positions of its poles manifest. In the case of the double gamma function, this representation is 

$\Gamma _{2}(w\mid a_{1},a_{2})={\frac {e^{\lambda _{1}w+\lambda _{2}w^{2}}}{w}}\prod _{\begin{array}{c}(n_{1},n_{2})\in \mathbb {N} ^{2}\\(n_{1},n_{2})\neq (0,0)\end{array}}{\frac {e^{{\frac {w}{n_{1}a_{1}+n_{2}a_{2}}}-{\frac {1}{2}}{\frac {w^{2}}{(n_{1}a_{1}+n_{2}a_{2})^{2}}}}}{1+{\frac {w}{n_{1}a_{1}+n_{2}a_{2}}}}}\ ,$

where we define the $w$ -independent coefficients

$\lambda _{1}=-{\underset {s=1}{\operatorname {Res} _{0}}}\zeta _{2}(s,0\mid a_{1},a_{2})\ ,$
$\lambda _{2}={\frac {1}{2}}{\underset {s=2}{\operatorname {Res} _{0}}}\zeta _{2}(s,0\mid a_{1},a_{2})+{\frac {1}{2}}{\underset {s=2}{\operatorname {Res} _{1}}}\zeta _{2}(s,0\mid a_{1},a_{2})\ ,$

where ${\underset {s=s_{0}}{\operatorname {Res} _{n}}}f(s)={\frac {1}{2\pi i}}\oint _{s_{0}}(s-s_{0})^{n-1}f(s)\,ds$  is an $n$ -th order residue at $s_{0}$ .

## Reduction to the Barnes G-function

The double gamma function with parameters $1,1$  obeys the relations 

$\Gamma _{2}(w+1|1,1)={\frac {\sqrt {2\pi }}{\Gamma (w)}}\Gamma _{2}(w|1,1)\quad ,\quad \Gamma _{2}(1|1,1)={\sqrt {2\pi }}\ .$

It is related to the Barnes G-function by

$\Gamma _{2}(w|1,1)={\frac {(2\pi )^{\frac {w}{2}}}{G(w)}}\ .$

## The double gamma function and conformal field theory

For $\Re b>0$  and $Q=b+b^{-1}$ , the function

$\Gamma _{b}(w)={\frac {\Gamma _{2}(w\mid b,b^{-1})}{\Gamma _{2}\left({\frac {Q}{2}}\mid b,b^{-1}\right)}}\ ,$

is invariant under $b\to b^{-1}$ , and obeys the relations

$\Gamma _{b}(w+b)={\sqrt {2\pi }}{\frac {b^{bw-{\frac {1}{2}}}}{\Gamma (bw)}}\Gamma _{b}(w)\quad ,\quad \Gamma _{b}(w+b^{-1})={\sqrt {2\pi }}{\frac {b^{-b^{-1}w+{\frac {1}{2}}}}{\Gamma (b^{-1}w)}}\Gamma _{b}(w)\ .$

For $\Re w>0$ , it has the integral representation

$\log \Gamma _{b}(w)=\int _{0}^{\infty }{\frac {dt}{t}}\left[{\frac {e^{-wt}-e^{-{\frac {Q}{2}}t}}{(1-e^{-bt})(1-e^{-b^{-1}t})}}-{\frac {\left({\frac {Q}{2}}-w\right)^{2}}{2}}e^{-t}-{\frac {{\frac {Q}{2}}-w}{t}}\right]\ .$

From the function $\Gamma _{b}(w)$ , we define the double Sine function $S_{b}(w)$  and the Upsilon function $\Upsilon _{b}(w)$  by

$S_{b}(w)={\frac {\Gamma _{b}(w)}{\Gamma _{b}(Q-w)}}\quad ,\quad \Upsilon _{b}(w)={\frac {1}{\Gamma _{b}(w)\Gamma _{b}(Q-w)}}\ .$

These functions obey the relations

$S_{b}(w+b)=2\sin(\pi bw)S_{b}(w)\quad ,\quad \Upsilon _{b}(w+b)={\frac {\Gamma (bw)}{\Gamma (1-bw)}}b^{1-2bw}\Upsilon _{b}(w)\ ,$

plus the relations that are obtained by $b\to b^{-1}$ . For $0<\Re w<\Re Q$  they have the integral representations

$\log S_{b}(w)=\int _{0}^{\infty }{\frac {dt}{t}}\left[{\frac {\sinh \left({\frac {Q}{2}}-w\right)t}{2\sinh \left({\frac {1}{2}}bt\right)\sinh \left({\frac {1}{2}}b^{-1}t\right)}}-{\frac {Q-2w}{t}}\right]\ ,$
$\log \Upsilon _{b}(w)=\int _{0}^{\infty }{\frac {dt}{t}}\left[\left({\frac {Q}{2}}-w\right)^{2}e^{-t}-{\frac {\sinh ^{2}{\frac {1}{2}}\left({\frac {Q}{2}}-w\right)t}{\sinh \left({\frac {1}{2}}bt\right)\sinh \left({\frac {1}{2}}b^{-1}t\right)}}\right]\ .$

The functions $\Gamma _{b},S_{b}$  and $\Upsilon _{b}$  appear in correlation functions of two-dimensional conformal field theory, with the parameter $b$  being related to the central charge of the underlying Virasoro algebra. In particular, the three-point function of Liouville theory is written in terms of the function $\Upsilon _{b}$ .