# An integral representation of the Riemann zeta function

I am referring to the equality in equation $3.29$ (page 12) and $4.20$ (page 17) in this paper.

I am unable to recognize where this comes from or what is the general expression for values other than $3$.

I checked at some online reviews like this - http://www.math.utah.edu/~milicic/zeta.pdf but nothing seems to match.

It would be great if someone can help.

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Where did you come across this? –  Steven Landsburg Aug 28 '13 at 20:51
Use the integral representations of $\zeta(s)\Gamma(s)$ for $s=3$. –  Dietrich Burde Aug 28 '13 at 21:28
The equation is incorrect as stated; you want $\zeta$, not $\xi$. But be that as it may, asking us where it comes from without telling us what you already know is a really good way to get your question closed. –  Steven Landsburg Aug 28 '13 at 21:38
"I came across" is a poor introduction. Where did you see this? Context? Of course some (possibly corrected) version can be adduced from known things, etc., but ... –  paul garrett Aug 28 '13 at 23:27
Why not actually put the equation here in the post? Asking people to go dig through a linked PDF is perhaps asking too much. –  Scott Morrison Sep 3 '13 at 0:32

you ask for the "general expression" for values other than $q=3$:
$$\int_{0}^{\infty}d\lambda\frac{\lambda^{q/2-1}}{1+e^{2 \pi \sqrt{\lambda}}}=2^{1-2q}(2^q-2)\pi^{-q}\Gamma(q) \zeta(q),\text{ for Re }q>0$$
$$\int_{0}^{\infty}d\lambda\frac{\lambda^{q-1}{\rm coth}(\pi\lambda)}{1+e^{2 \pi \lambda}}=(2\pi)^{-q}\Gamma(q) \zeta(q),\text{ for Re }q>1$$