[SOLVED] Fourier series 1. The problem statement, all variables and given/known data Hi. Please take a look at: https://www.physicsforums.com/showthread.php?t=104306 It is part 1 (finding the Fourier-series for f(x) = |x|) I am having trouble with. 3. The attempt at a solution Ok, I can write f(x) = |x| for -pi =< x < pi as: f(x) = x for 0 =< x < pi and f(x) = -x for -pi =< x =< 0. The function has period T=2. I want to find a_n = int[f(x)*cos(n*pi*x)]dx with limits -1 .. 1. I split this integral up in two: a_0 = int[-x}dx + ... but what are the limits? Are they -pi..0 or -1..0? The way I see it, neither of the limits work since a_0 must be pi.