Primality proof for n = 3919:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=653.html>653 is prime.</a>
<br>b^((n-1)/653)-1 mod n = 63, which is a unit, inverse 1804.
<p>(653) divides n-1.
<p>(653)^2 > n.
<p>n is prime by Pocklington's theorem.