> I think this example is well known but I can’t think what it is Could be the basel problem though he probably used this trick many times in his prolific life

> I think this example is well known but I can’t think what it is Could be the basel problem though he probably used this trick many times in his prolific life: https://en.wikipedia.org/wiki/Basel_problem#Euler's_approach https://en.wikipedia.org/wiki/Basel_problem#Euler's_approach