Euler's first major work, the one that brought him fame, was his solution to the Basel problem (https://en.wikipedia.org/wiki/Basel_problem https://en.wikipedia

Euler's first major work, the one that brought him fame, was his solution to the Basel problem (https://en.wikipedia.org/wiki/Basel_problem https://en.wikipedia.org/wiki/Basel_problem), when he was 28. Namely, he found the sum of the reciprocals of the squares of the integer, i.e. the value of the series (1 + 1/4 + 1/9 + 1/16 + ...), to be π²/6. According to https://faculty.math.illinois.edu/~reznick/sandifer.pdf https://faculty.math.illinois.edu/~reznick/sandifer.pdf Euler gave three solutions in his 1736 paper now numbered E41: http://eulerarchive.maa.org/pages/E041.html http://eulerarchive.maa.org/pages/E041.html ("De summis serierum reciprocarum" = "On the sums of series of reciprocals") Here is Jordan Bell's translation: https://arxiv.org/abs/math/0506415 https://arxiv.org/abs/math/0506415