Multiple gamma function

Definition

For \Re a_i0, 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_ia_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-\frac12}}{\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)\ .

In the case of the double Gamma function, the asymptotic behaviour for w\to \infty is known, and the leading factor is : \Gamma_2(w|a_1,a_2)\ \underset{w\to \infty}{\sim}\ w^{\frac{w^2}{2a_1a_2}} \quad \text{for}\quad \left{\begin{array}{l} \frac{a_1}{a_2}\in\mathbb{C}\backslash(-\infty,0]\ , \ w \in \mathbb{C}\backslash \left(\mathbb{R}+a_1+\mathbb{R}+a_2\right)\ . \end{array}\right.

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_1w +\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_1a_1+n_2a_2}- \frac12 \frac{w^2}{(n_1a_1+n_2a_2)^2}}}{1+\frac{w}{n_1a_1+n_2a_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 = \frac12\underset{s=2}{\operatorname{Res}_0}\zeta_2(s,0\mid a_1,a_2) + \frac12 \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.

Another representation as a product over \mathbb{N} leads to an algorithm for numerically computing the double Gamma function.

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|\alpha,\alpha) = (2\pi)^\frac{w}{2\alpha} \alpha^{-\frac{w^2}{2\alpha^2} + \frac{w}{\alpha} - 1} G(w / \alpha)^{-1} \ .

The double gamma function and conformal field theory

For \Re b0 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-\frac12}}{\Gamma(bw)}\Gamma_b(w)\quad , \quad \Gamma_b(w+b^{-1}) = \sqrt{2\pi}\frac{b^{-b^{-1}w+\frac12}}{\Gamma(b^{-1}w)} \Gamma_b(w)\ .

For \Re w0, 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 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(\frac12 bt\right)\sinh\left(\frac12 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)^2e^{-t} -\frac{\sinh^2\frac12\left(\frac{Q}{2}-w\right)t}{\sinh\left(\frac12 bt\right)\sinh\left(\frac12 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.

References

References

1. (2009). "On the Barnes double zeta and gamma functions". Journal of Number Theory.
3. "Recent progress on Liouville Field Theory".