Primality proof for n = 3969899:

Take b = 2.

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

104471 is prime.
b^((n-1)/104471)-1 mod n = 2100183, which is a unit, inverse 2243650.

(104471) divides n-1.

(104471)^2 > n.

n is prime by Pocklington's theorem.