Primality proof for n = 28687:

Take b = 2.

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

683 is prime.
b^((n-1)/683)-1 mod n = 26969, which is a unit, inverse 13542.

(683) divides n-1.

(683)^2 > n.

n is prime by Pocklington's theorem.