Primality proof for n = 1276987:

Take b = 2.

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

179 is prime.
b^((n-1)/179)-1 mod n = 449904, which is a unit, inverse 486054.

41 is prime.
b^((n-1)/41)-1 mod n = 1207307, which is a unit, inverse 543984.

(41 * 179) divides n-1.

(41 * 179)^2 > n.

n is prime by Pocklington's theorem.