Multiple gamma function

For derivatives of the log of the gamma function, see polygamma 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

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

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.

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

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

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

where we define the $w$-independent coefficients

$\lambda _{1}=-{\underset {s=1}{\mathrm {Res} _{0}}}\zeta _{2}(s,0|a_{1},a_{2})\ ,$

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

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

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, 66, The Royal Society: 265–268, doi:10.1098/rspl.1899.0101, ISSN 0370-1662, JSTOR 116064
• 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, 196, The Royal Society: 265–387, Bibcode:1901RSPTA.196..265B, doi:10.1098/rsta.1901.0006, ISSN 0264-3952, JSTOR 90809
• Barnes, E. W. (1904), "On the theory of the multiple gamma function", Trans. Camb. Philos. Soc., 19: 374–425
• Friedman, Eduardo; Ruijsenaars, Simon (2004), "Shintani–Barnes zeta and gamma functions", Advances in Mathematics, 187 (2): 362–395, doi:10.1016/j.aim.2003.07.020, ISSN 0001-8708, MR 2078341
• Ruijsenaars, S. N. M. (2000), "On Barnes' multiple zeta and gamma functions", Advances in Mathematics, 156 (1): 107–132, doi:10.1006/aima.2000.1946, ISSN 0001-8708, MR 1800255
• Ponsot, B., Recent progress on Liouville Field Theory, arXiv:hep-th/0301193, arXiv:hep-th/0301193, Bibcode:2003PhDT.......180P