Primality proof for n = 281:
<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 = 248, which is a unit, inverse 17.
<p><a href=5.html>5 is prime.</a>
<br>b^((n-1)/5)-1 mod n = 231, which is a unit, inverse 118.
<p>(5 * 7) divides n-1.
<p>(5 * 7)^2 > n.
<p>n is prime by Pocklington's theorem.