Cyclotomic Polynomials
An applet to visualize the roots of unity for the nth cyclotomic polynomial.
The nth cyclotomic polynomial is the largest degree factor of x^n-1 which divides no x^m-1 for m<n. You have to put it that way, because if a prime p divides n, then the pth cyclotomic divides x^n-1. Anyhow, the result of this is that the roots of the nth cyclotomic correlate to the fractions i/n that are irreducible.