Integral expression for zeta(2) - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T02:15:07Z http://mathoverflow.net/feeds/question/14374 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/14374/integral-expression-for-zeta2 Integral expression for zeta(2) Franz Lemmermeyer 2010-02-06T11:15:41Z 2010-02-06T12:11:05Z <p>By computing the sum of all Bernoulli numbers via Borel summation (I learned this technique from Varadarajan's excellent book <em>Euler through time. A new look at old themes</em>, 2006) I found that $$\sum B_n = \int_0^\infty \frac{t}{e^{2t}-e^t} dt$$ and discovered numerically that this expression equals $\zeta(2)-1$. The web is not very good for finding out where this can be found in print. Where should I look, and how can equations such as $$\zeta(2) = 1 + \int_0^\infty \frac{t}{e^{2t}-e^t}\ dt$$ be proved?</p> http://mathoverflow.net/questions/14374/integral-expression-for-zeta2/14381#14381 Answer by engelbrekt for Integral expression for zeta(2) engelbrekt 2010-02-06T12:11:05Z 2010-02-06T12:11:05Z <p>The starting point is the integral </p> <p>$$\Gamma(s) = \int_{0}^{\infty}e^{-x}x^{s-1}dx$$</p> <p>for the gamma function. Make the change of variable $x = nu$ with $n$ an arbitrary positive integer. Then </p> <p>$$\Gamma(s)n^{-s} = \int_{0}^{\infty}e^{-nu}u^{s-1}du$$</p> <p>and summing over $n$ from $n = 1$ yields</p> <p>$$\Gamma(s)\zeta(s) = \int_0^{\infty}\frac{1}{e^u - 1}u^{s-1}du.$$</p> <p>This formula was the starting point of one of Riemann's two proofs of the functional equation. I am not certain who discovered it first, but it may have been Abel.</p> <p>Substituting $s = 2$ gives </p> <p>$$\zeta(2) = \int_{0}^{\infty}\frac{u}{e^u - 1}du$$</p> <p>and so</p> <p>$$\zeta(2) = \int_{0}^{\infty}\frac{ue^u}{e^{2u} - e^u}du = \int_{0}^{\infty}\frac{u(e^u - 1) + u}{e^{2u} - e^u}du = \int_{0}^{\infty}\left(ue^{-u} + \frac{u}{e^{2u} - e^u}\right)du = 1 + \int_{0}^{\infty}\frac{u}{e^{2u} - e^u}du.$$</p>