Primality proof for n = 278486521:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=10501.html>10501 is prime.</a>
<br>b^((n-1)/10501)-1 mod n = 131644513, which is a unit, inverse 121741978.
<p><a href=17.html>17 is prime.</a>
<br>b^((n-1)/17)-1 mod n = 262216189, which is a unit, inverse 84587057.
<p>(17 * 10501) divides n-1.
<p>(17 * 10501)^2 > n.
<p>n is prime by Pocklington's theorem.