Primality proof for n = 1206781:

Take b = 2.

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

20113 is prime.
b^((n-1)/20113)-1 mod n = 78447, which is a unit, inverse 709359.

(20113) divides n-1.

(20113)^2 > n.

n is prime by Pocklington's theorem.