Primality proof for n = 246608641:

Take b = 2.

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

21407 is prime.
b^((n-1)/21407)-1 mod n = 170583700, which is a unit, inverse 96349255.

(21407) divides n-1.

(21407)^2 > n.

n is prime by Pocklington's theorem.