User:Michael Hardy/Petry on Euler's zeta function

Posted by David Petry on July 15th, 1995:

Subject: Euler's zeta function

Note that

$\sin \left(n\arcsin \left({\frac {x}{n}}\right)\right)$

is a polynomial for odd n.

[ ... ]

the power series for it starts off

$x-{\frac {1-{\frac {n^{2}}{6}}}{6}}x^{3}+\cdots .$

[ ... ]

the roots of the polynomial are:

$n\sin \left({\frac {m\pi }{n}}\right){\text{ for }}{\frac {-n}{2}}<m<{\frac {n}{2}}$

and we can factor the polynomial as

$x\prod _{i}\left(1-{\frac {x^{2}}{r_{i}^{2}}}\right)$

where r_i are the positive roots.

Equating the power series and the expansion of the product gives

${\frac {1-{\frac {1}{n^{2}}}}{6}}=\sum _{m=1}^{(n-1)/2}{\frac {1}{\left(n\sin \left({\frac {m\pi }{n}}\right)\right)^{2}}}$

and note that

$\lim _{n\to \infty }n\sin \left({\frac {m\pi }{n}}\right)=m\pi$

for any fixed m.

and with just a little more work we have the desired result!