Fourier series, from the heat equation to sines to cycles.
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12 minutes of pure Fourier series animations: https://youtu.be/-qgreAUpPwM

Some viewers made apps that create circle animations for your own drawing.  Check them out!
https://www.reddit.com/r/3Blue1Brown/comments/cvpdn7/make_your_own_fourier_circle_drawings/
https://isaacvr.github.io/coding/fourier_transform/

Thanks to Stuart@Biocinematics for the one-line sketch of Fourier via twitter.  As it happens, he also has an educational YouTube channel:
https://www.youtube.com/channel/UCKOiJd9YCbv7LeL2LFOGiLQ

Small correction: at 9:33, all the exponents should have a pi^2 in them.

If you're looking for more Fourier Series content online, including code to play with to create this kind of animation yourself, check out these posts:

Mathologer
https://youtu.be/qS4H6PEcCCA

The Coding Train
https://youtu.be/Mm2eYfj0SgA

Jezmoon
http://www.jezzamon.com/fourier/index.html

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