Take b = 2.

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

855109789 is prime.
b^((n-1)/855109789)-1 mod n = 1580317812275, which is a unit, inverse 752553502718.

(855109789) divides n-1.

(855109789)^2 > n.

n is prime by Pocklington's theorem.