Take b = 2.

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

19 is prime.

b^((n-1)/19)-1 mod n = 4063, which is a unit, inverse 3633.

13 is prime.

b^((n-1)/13)-1 mod n = 1746, which is a unit, inverse 5162.

(13 * 19) divides n-1.

(13 * 19)^2 > n.

n is prime by Pocklington's theorem.