Activity: Fourier Transform of a Derivative

Periodic Systems 2022

The formula for the inverse Fourier transform shows that a function \(f(x)\) can be written in terms of its Fourier transform via \begin{equation} f(x)= \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} \tilde{f}(k)\, e^{ikx}\, dk \end{equation} Take the derivative of both sides of this equation with respect to \(x\) and simplify. Interpret your expression as the inverse Fourier transform of something.

Instructor's Guide

Introduction

Students will need a short lecture giving the definition of the inverse Fourier Transform \begin{equation} {\cal{F}}^{-1}(\tilde{f}) =f(x)= \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} f(k)\, e^{ikx}\, dk \end{equation}

Student Conversations

The logic of this problem may feel a little backwards to students. Be prepared to be more directive than normal in helping the groups that get stuck. Or consider doing this problem as a mini-lecture, rather than a group activity, especially if time is tight.

Wrap-up

The result if this calculation is an essential formula in solving differential equations with Fourier transforms.

Learning Outcomes