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 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|a_{1},...,a_{N})=\exp \left(\left.{\frac {\partial }{\partial s}}\zeta _{N}(s,w|a_{1},...,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 $w$ , $\Gamma _{N}(w|a_{1},...,a_{N})$ 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, $\Gamma _{N}(w|a_{1},...,a_{N})$ is the unique meromorphic function of finite order with these zeros and poles.

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

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|b,b^{-1})}{\Gamma _{2}\left({\frac {Q}{2}}|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 $\Gamma _{b}(w)$ , it is possible to define the two functions

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 $\Upsilon _{b}$ .

References

Barnes, E. W. (1899), "The Genesis of the Double Gamma Functions", Proc. London Math. Soc., s1-31: 358–381, doi:10.1112/plms/s1-31.1.358
Barnes, E. W. (1899), "The Theory of the Double Gamma Function. [Abstract]", Proceedings of the Royal Society of London, The Royal Society, 66: 265–268, ISSN 0370-1662, JSTOR 116064, doi:10.1098/rspl.1899.0101
Barnes, E. W. (1901), "The Theory of the Double Gamma Function", Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character, The Royal Society, 196: 265–387, ISSN 0264-3952, JSTOR 90809, doi:10.1098/rsta.1901.0006
Barnes, E. W. (1904), "On the theory of the multiple gamma function", Trans. Cambridge Philos. Soc., 19: 374–425
Friedman, Eduardo; Ruijsenaars, Simon (2004), "Shintani–Barnes zeta and gamma functions", Advances in Mathematics, 187 (2): 362–395, ISSN 0001-8708, MR 2078341, doi:10.1016/j.aim.2003.07.020
Ruijsenaars, S. N. M. (2000), "On Barnes' multiple zeta and gamma functions", Advances in Mathematics, 156 (1): 107–132, ISSN 0001-8708, MR 1800255, doi:10.1006/aima.2000.1946
Ponsot, B., Recent progress on Liouville Field Theory, arXiv:hep-th/0301193

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