Comment by rockmeamedee

I had no idea, but this "wiggle" is required for an optimal approximation, it's called the "equioscillation property" [https://en.wikipedia.org/wiki/Equioscillation_theorem].

For a polynomial P (of degree n) to approximate a function F on the real numbers with minimal absolute error, the max error value of |P - F| needs to be hit multiple times, (n+2 times to be precise). You need to have the polynomial "wiggle" back and forth between the top of the error bound and the bottom.

And even more surprisingly, this is a necessary _and sufficient_! condition for optimality. If you find a polynomial whose error alternates and it hits its max error bound n+2 times, you know that no other polynomial of degree n can do better, that is the best error bound you can get for degree n.

Very cool!