Primality proof for n = 521:
<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 = 254, which is a unit, inverse 80.
<p><a href=5.html>5 is prime.</a>
<br>b^((n-1)/5)-1 mod n = 515, which is a unit, inverse 434.
<p>(5 * 13) divides n-1.
<p>(5 * 13)^2 > n.
<p>n is prime by Pocklington's theorem.