Primality proof for n = 1621:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=5.html>5 is prime.</a>
<br>b^((n-1)/5)-1 mod n = 1488, which is a unit, inverse 195.
<p><a href=3.html>3 is prime.</a>
<br>b^((n-1)/3)-1 mod n = 1435, which is a unit, inverse 61.
<p>(3^4 * 5) divides n-1.
<p>(3^4 * 5)^2 > n.
<p>n is prime by Pocklington's theorem.