Primality proof for n = 251969987:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=397429.html>397429 is prime.</a>
<br>b^((n-1)/397429)-1 mod n = 20517413, which is a unit, inverse 91928706.
<p>(397429) divides n-1.
<p>(397429)^2 > n.
<p>n is prime by Pocklington's theorem.