The most intuitive, easy-to-follow explanation of Fourier transforms I've ever come across
It's here. I found the link when I happened across a discussion of the topic in the Math Stack Exchange. I wish I had seen this way back when I was implementing FFTs in my computer music class in college. It would have saved me some headaches.
Bonus interesting math thing: The Genuine Sieve of Eratosthenes, a paper by Melissa E. O’Neill.