Primality proof for n = 20599:

Take b = 2.

b^(n-1) mod n = 1.

3433 is prime.
b^((n-1)/3433)-1 mod n = 63, which is a unit, inverse 10463.

(3433) divides n-1.

(3433)^2 > n.

n is prime by Pocklington's theorem.