Primality proof for n = 20521:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=19.html>19 is prime.</a>
<br>b^((n-1)/19)-1 mod n = 11759, which is a unit, inverse 9230.
<p><a href=5.html>5 is prime.</a>
<br>b^((n-1)/5)-1 mod n = 7807, which is a unit, inverse 15971.
<p><a href=3.html>3 is prime.</a>
<br>b^((n-1)/3)-1 mod n = 515, which is a unit, inverse 13508.
<p>(3^3 * 5 * 19) divides n-1.
<p>(3^3 * 5 * 19)^2 > n.
<p>n is prime by Pocklington's theorem.