Take b = 2.

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

19 is prime.

b^((n-1)/19)-1 mod n = 1435, which is a unit, inverse 595.

13 is prime.

b^((n-1)/13)-1 mod n = 2671, which is a unit, inverse 318.

(13 * 19) divides n-1.

(13 * 19)^2 > n.

n is prime by Pocklington's theorem.