1. The problem statement, all variables and given/known data I have the Fourier-series for the function: f(x) = x for -Pi < x < Pi. I know wish to find the Fourier-series for the function: g(x) = x for -p < x < p, where p is a random period. 3. The attempt at a solution Ok, the obvious thing here is to use the result from f(x). If thought that if I define f(p*x/Pi) = g(x) = f(y), then I just insert x=y*Pi/p. But in the result of this exercise, they have an additional p/Pi in front of the sum. Where does this come from?