Primality proof for n = 821:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=41.html>41 is prime.</a>
<br>b^((n-1)/41)-1 mod n = 158, which is a unit, inverse 556.
<p>(41) divides n-1.
<p>(41)^2 > n.
<p>n is prime by Pocklington's theorem.