Primality proof for n = 216558437:

Take b = 2.

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

1746439 is prime.
b^((n-1)/1746439)-1 mod n = 17781006, which is a unit, inverse 29983791.

(1746439) divides n-1.

(1746439)^2 > n.

n is prime by Pocklington's theorem.