## Only for the elite of Mathematics

Analisi, algebra lineare, topologia, gruppi, anelli, campi, ...
marcomila99
Messaggi: 4
Iscritto il: 23 giu 2017, 19:42

### Only for the elite of Mathematics

How do you prove this $\displaystyle z\cot \left(z\right)=1-2\sum _{k=1}^{\infty}\frac{z^2}{k^2\pi ^2-z^2}$ ?

i’ve seen this formula on a document about Bernoulli’s numbers and their relations with Riemann’s zeta function (http://www.luciocadeddu.com/tesi/Aru_magistrale.pdf corollario 2.5.2) but i really don’t understand why this is true…

<enigma>
Messaggi: 876
Iscritto il: 24 set 2009, 16:44

### Re: Only for the elite of Mathematics

I haven't looked at the document you link to, but the standard way to treat this kind of problems is to combine the Cauchy integral formula and the Phragmén–Lindelöf principle to get an appropriate residue theorem for unbounded domains. For this specific identity I believe there also is a more ad hoc technique known as the Herglotz trick.
"Quello lì pubblica come un riccio!" (G.)
"Questo puoi mostrarlo o assumendo abc o assumendo GRH+BSD, vedi tu cos'è meno peggio..." (cit.)