# Fermat pseudoprime

• Apr 16th 2008, 01:37 AM
milfner
Fermat pseudoprime
(Angry)
• May 2nd 2008, 09:45 PM
jtsab
For the 1st one, its easy to see that $91 = 7 * 13$ so its definitely not prime.

Also $3^{90} = 1mod\ 90$ (from observation)

You can also construct a new pseudoprime N using the proposition:

Let b >= 2 (where b is the base of this pseudoprime) and p an odd prime where p does not divide b(b-1)(b+1) then

$N = \frac{b^p -1}{b-1}*\frac{b^p+1}{b+1}$

If someone could review this and check for correctness, that would be great! :)
Edit: it works fine.
• May 2nd 2008, 10:15 PM
milfner
(Angry)