The Basel problem is an all-time classic math problem. Among all of its solutions, my favorite is based around the computation of a double integral. Here is a link to the source of this solution: In fact, using the same technique from the video, one can evaluate the infinite sum of 1/n^(2k) for any (positive integer) k! A slightly different double integral solution can be found here: Obviously, more reading about the problem can be found here: