Primality proof for n = 643:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=107.html>107 is prime.</a>
<br>b^((n-1)/107)-1 mod n = 63, which is a unit, inverse 296.
<p>(107) divides n-1.
<p>(107)^2 > n.
<p>n is prime by Pocklington's theorem.