1. The problem statement, all variables and given/known data This is a question regarding Fourier series. ∫ dx |f(x)|^2 = ∑ |Cn|2 (note the integral is between -π and π, and the sum is from n= -∞ to ∞) 2. Relevant equations Complex Fourier series: f(x) = ∑Cn einx (again between n = -∞ and ∞) 3. The attempt at a solution So I figured the complex conjugate of the Fourier transform would be: f*(x)= ∑ C*n e-inx so |f(x)|2 = ∑ |Cn|2 (as the exponentials cancel) But I don't understand how the integral comes into things? I think I'm just not overly good at manipulating summations etc, so any help would be greatly appreciated! Thanks.