Primality proof for n = 248431:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=13.html>13 is prime.</a>
<br>b^((n-1)/13)-1 mod n = 233885, which is a unit, inverse 175999.
<p><a href=7.html>7 is prime.</a>
<br>b^((n-1)/7)-1 mod n = 113350, which is a unit, inverse 149429.
<p>(7^2 * 13^2) divides n-1.
<p>(7^2 * 13^2)^2 > n.
<p>n is prime by Pocklington's theorem.