Take b = 2.

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

821796863 is prime.
b^((n-1)/821796863)-1 mod n = 424512317759444, which is a unit, inverse 33078999059648.

(821796863) divides n-1.

(821796863)^2 > n.

n is prime by Pocklington's theorem.