# Fourier Series of Secant

by mathskier
Tags: fourier, secant, series
 P: 30 So I know that sec(x) has period 2Pi, and it's even so I don't need to figure out coefficients for bn. Let's take the limits of the integral to go from -3/2 Pi to 1/2 Pi. How do I integrate sec(x) sin(nx) dx?! Am I on the right path? PS: I know that this doesn't satisfy the Dirichlet Theorem, but the textbook I'm using still says to compute it.
 Homework Sci Advisor HW Helper Thanks ∞ P: 9,644 Seems to me that integral is zero for n even and indeterminate for n odd.
 P: 2,251 well since $\sec(x) = 1/\cos(x)$ then i think you can express the cosine functions, both on the top and on the bottom as exponentials with imaginary argument. i think you can get to an integral that looks like $$c_n = \frac{1}{2 \pi} \int_{-\pi}^{+\pi} \frac{2}{e^{i (n+1) x}+e^{i (n-1) x}} \ dx$$ i think you can find an antiderivative of that. might still come out as indeterminate. but that's the integral you have to solve.
Emeritus
HW Helper
Thanks
PF Gold
P: 11,670
Fourier Series of Secant

 Quote by haruspex Seems to me that integral is zero for n even and indeterminate for n odd.
I evaluated the first few odd n integrals using Mathematica, and it looks like ##a_{2n+1} = (-1)^n 2\pi##.
Homework
HW Helper
Thanks ∞
P: 9,644
 Quote by vela I evaluated the first few odd n integrals using Mathematica, and it looks like ##a_{2n+1} = (-1)^n 2\pi##.
The reason I thought it indeterminate is that integrating over a 2π range effectively cancels a +∞ with a -∞. E.g. with n=1, it's the integral of tan, which is log cos. An interval that spans π/2 will appear to give a sensible answer, but in reality it's undefined.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,670 mathskier indicated the wrong integrand. It should be sec x cos(nx), which is the integrand I used.
Homework