Basel Problem

Theorem

 * $\displaystyle \zeta \left({2}\right) = \sum_{n \mathop = 1}^{\infty} {\frac 1 {n^2}} = \frac {\pi^2} 6$

where $\zeta$ denotes the Riemann zeta function.

Historical Note
Although often credited to Euler, it has been suggested that it was in fact solved by Nicolaus I Bernoulli.