Basel Problem

Theorem

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

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.