Basel P.

The Basel Problem: sum of inverse squares on consecutive numbers.
This comes to (pi^2)/6, and this precise number was the work of Euler.
Basel was the home town of the Bernoullis, who first grappled with
the problem. Johann Bernoulli tutored Euler in mathematics.

This is quite interesting because it is precisely finding and proving this
that led Euler to first propose the Zeta function; and its infamous descendant
for us, the Rieman Zeta function on the complex plane.

One can see easily enough that for exponents 0 and 1, the series is infinite.
The problem with squares and higher is to find the convergence formula.

                                            source: Gerard Villemin

                                                             *     *     *
Went to Wikipedia to help me through a proof for the Basel problem. The
starred solution was in Russian! Below, in machine translation to English:

https://www.desmos.com/calculator/yu7h2cqcnl