Primality proof for n = 166021:

Take b = 2.

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

2767 is prime.
b^((n-1)/2767)-1 mod n = 141431, which is a unit, inverse 162787.

(2767) divides n-1.

(2767)^2 > n.

n is prime by Pocklington's theorem.