Recently, a question was asked on the site on how to approximate some quantity (either integral or series; I forget and am no longer following the post). But one of the people who provided a good answer used some result on the sum of Fourier coefficients. I wanted to ask if there is a theorem about sum of Fourier coefficients. I can't seem to find anything online.
Of course, I'm not referring to Parseval's identity, as that is for the sum of the squares of the coefficients. Thank you.