Barnes integral

In mathematics, a Barnes integral or Mellin-Barnes integral is a contour integral involving a product of gamma functions. It is named for Ernest William Barnes.

Hypergeometric series

The hypergeometric function is given as a Barnes integral (Barnes 1908) by

${}_{2}F_{1}(a,b;c;z)={\frac {\Gamma (c)}{\Gamma (a)\Gamma (b)}}{\frac {1}{2\pi i}}\int _{-i\infty }^{i\infty }{\frac {\Gamma (a+s)\Gamma (b+s)\Gamma (-s)}{\Gamma (c+s)}}(-z)^{s}ds$

Barnes lemmas

The first Barnes lemma (Barnes 1908) states

${\frac {1}{2\pi i}}\int _{-i\infty }^{i\infty }\Gamma (a+s)\Gamma (b+s)\Gamma (c-s)\Gamma (d-s)ds={\frac {\Gamma (a+c)\Gamma (a+d)\Gamma (b+c)\Gamma (b+d)}{\Gamma (a+b+c+d)}}$

This is an analogue of Gauss's ₂F₁ summation formula, and also an extension of Euler's beta integral. The integral in it is sometimes called Barnes's beta integral.

The second Barnes lemma (Barnes 1910) states

${\frac {1}{2\pi i}}\int _{-i\infty }^{i\infty }{\frac {\Gamma (a+s)\Gamma (b+s)\Gamma (c+s)\Gamma (1-d-s)\Gamma (-s)}{\Gamma (e+s)}}ds$

$={\frac {\Gamma (a)\Gamma (b)\Gamma (c)\Gamma (1-d+a)\Gamma (1-d+b)\Gamma (1-d+c)}{\Gamma (e-a)\Gamma (e-b)\Gamma (e-c)}}$

where e = a + b + c − d + 1. This is an analogue of Saalschutz's summation formula.

References

• Barnes, E. W. (1908) A New Development in the Theory of the Hypergeometric Functions Proc. London Math. Soc. 6, 141-177.