Primality proof for n = 647:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=19.html>19 is prime.</a>
<br>b^((n-1)/19)-1 mod n = 543, which is a unit, inverse 591.
<p><a href=17.html>17 is prime.</a>
<br>b^((n-1)/17)-1 mod n = 292, which is a unit, inverse 534.
<p>(17 * 19) divides n-1.
<p>(17 * 19)^2 > n.
<p>n is prime by Pocklington's theorem.