Consider the sequence $(a_n)_{n \in \N }$ defined by \[ a_n = \frac{1}{n^2}. \] Does $\lim_{N \to \infty} \sum_{n = 1}^{N} a_n$ exist? If so, what is the limit? These questions are known as the Basel problem.
\[ a_n = \frac{1}{n^2}. \]
\[ \lim_{N \to \infty} \sum_{n = 1}^{N} s_n = \frac{\pi ^2}{6}. \]