Nb. He's just using the N = 2 DFT to solve a quadratic equation. This approach is well-understand in the context of Galois theory, eg. the N = 3 DFT reduces th

Nb. He's just using the N = 2 DFT to solve a quadratic equation. This approach is well-understand in the context of Galois theory, eg. the N = 3 DFT reduces the general cubic to a set of at most quadratic equations, and in general, you can DFT your way down a Galois Group's composition series whenever it's solvable.