How many pseudoprimes are there?
It is important to know that there are infinitely many pseudoprimes to a given base. Specialists consider that there are only three pseudoprimes to base 2 below 1000, 245 below one million, and only 21853 less than 25·109 . Keep in mind that in number theory, the Fermat pseudoprimes make up the most important class of pseudoprimes that come from Fermat’s little theorem. Not too many people know that Pseudoprimes to base 2 are sometimes called Poulet numbers, after the Belgian mathematician Paul Poulet.