A Fermat number is a number of the form
for integers . A Fermat number that is prime is called a Fermat prime.
Pierre de Fermat conjectured (1640) that all are prime, but is composite (641 is a factor, discovered by Euler in 1732), and in fact (as of 2014) only for are known to be prime. Basic questions remain open, such as:
The Fermat numbers for are known to be composite, though in some cases no explicit factors are known (), or incomplete factorisations (). Isolated other cases are known to be composite, for instance .
There are heuristic (nonrigorous) arguments that there should be only finitely many Fermat primes, and that Fermat primes have positive density in the primes; it is impossible that both of these hold, since finite sets have zero density in a countable set like that of the primes. In any case, it is conjectured by various people that: there are only finitely many Fermat primes; there are only five Fermat primes; and that there are finitely many, but more than five (this last position is argued by Emil Artin).
The following page lists heuristic arguments as to the finiteness of the Fermat primes, and also that there are only five such:
New factors of composite Fermat numbers are announced here:
Last revised on August 8, 2018 at 11:29:29. See the history of this page for a list of all contributions to it.