Primality proof for n = 61681:
<p>Take b = 3.
<p>b^(n-1) mod n = 1.
<p><a href=257.html>257 is prime.</a>
<br>b^((n-1)/257)-1 mod n = 28921, which is a unit, inverse 54001.
<p>(257) divides n-1.
<p>(257)^2 > n.
<p>n is prime by Pocklington's theorem.