<mathematics> A prime number of the form 2^2^n + 1. Any prime number of
the form 2^n+1 must be a Fermat prime. Fermat conjectured in a letter to someone
or other that all numbers 2^2^n+1 are prime, having noticed that this is true
Euler proved that 641 is a factor of 2^2^5+1. Of course nowadays we would just
ask a computer, but at the time it was an impressive achievement (and his proof
is very elegant).
No further Fermat primes are known; several have been factorised, and several
more have been proved composite without finding explicit factorisations.
Gauss proved that a regular N-sided polygon can be constructed with ruler and
compasses if and only if N is a power of 2 times a product of distinct Fermat
fencepost error « fepped out « FEPROM « Fermat
» Ferranti F100-L » ferrite core memory »