1. The problem statement, all variables and given/known data Hi all. I have used Fourier series alot in my maths study, and now I am about to use it when analyzing electrical circuits. In my notes from a lecture, I wrote down an example done by the professor. We are looking at a RCL-circuit with an applied EMF given by the periodic square function, which is ε0 from 0 to T/2 and -ε0 from T/2 to T. And from this it is just periodic. We wish to find the current. Now we have found the complex Fourier series for the electromotive force, and we are able to write it as: ε(t) = -Im[ε'(t)], where Im is the imaginary part and ε' is some complex Fourier series. He then says that "Hence we can write I(t) = -Im[I'(t)]", and this is where I get confused. Why is it that we know that I(t) = -Im[I'(t)], just because ε(t) = -Im[ε'(t)]? I hope you guys can help me. I've thought about this most of the weekend, and the notes are very poor. Unfortunately Google doesn't help me either. Thanks in advance. Sincerely, Niles.