Primality proof for n = 491:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=7.html>7 is prime.</a>
<br>b^((n-1)/7)-1 mod n = 137, which is a unit, inverse 448.
<p>(7^2) divides n-1.
<p>(7^2)^2 > n.
<p>n is prime by Pocklington's theorem.