Suggest edit — SU-MATH53 MAR042024

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---
title: "SU-MATH53 MAR042024"
source: https://www.jemoka.com/posts/kbhsu_math53_mar042024/
date: 2024-03-04
---

What if, Fourier Series, but exponential?
This also motivates Discrete Fourier Transform.
Also Complex Exponential.
Review Recall again that if we have a periodic function, we&rsquo;ve got:
\begin{equation} f(x) = \sum_{k=0}^{\infty} a_{k} \sin \qty( \frac{2\pi k}{l} x) + b_{n} \cos \qty( \frac{2\pi k x}{L}) \end{equation}
We note that this breaks individually into the sign and cosine series depending of the function&rsquo;s oddness.
Complex Fourier Series This will begin by feeling like a notation rewrite:
\begin{equation} f(x) = \sum_{-\infty}^{\infty} c_{n} e^{n \omega x i} \end{equation}
where \(\omega = \frac{2\pi}{L}\).
Why is this summing from negative to positive?
Consider:
\begin{equation} \cos \qty(nx) = \frac{e^{inx}+e^{-inx}}{2} \end{equation}
You will note that summing \(n \in 0 &hellip; \infty\), plugging it into above, will result in summing from both \(n \in -\infty &hellip; \infty\).
Finding \(c_{n}\) Recall that complex exponentials are orthonormal + inner product over complex-valued functions
Because most cancels except one thing, we get:
\begin{equation} \langle f, e^{i\omega n x} \rangle = c_{n} L \end{equation}
meaning:
\begin{equation} c_{n} = \frac{1}{L} \int_{0}^{L} f(x) e^{-i\omega n x} \dd{x} = \frac{1}{L} \int_{\frac{-L}{2}}^{\frac{L}{2}} f(x) e^{-i\omega n x} \dd{x} \end{equation}
if our function is \(L\) periodic.
NOTE: this integral has a NEGATIVE power vs the series has a POSITIVE power!!
Complex Exponentials with Sawtooth Consider:
\begin{equation} f(x) = x-n \end{equation}
where this function is periodic over \(n \leq x \leq n+1\), so&mdash;
\begin{equation} c_{n} = \int_{0}^{1} x e^{-2\pi i n x} \dd{x} = -\frac{1}{2\pi i n} e^{-2 \pi i n} \end{equation}