Primality proof for n = 21871:
<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 = 16388, which is a unit, inverse 9322.
<p><a href=3.html>3 is prime.</a>
<br>b^((n-1)/3)-1 mod n = 11706, which is a unit, inverse 17968.
<p>(3^7 * 5) divides n-1.
<p>(3^7 * 5)^2 > n.
<p>n is prime by Pocklington's theorem.