Fermat prime
Fermat prime
(mathematics)Euler proved that 641 is a factor of 2^2^5+1. Of coursenowadays we would just ask a computer, but at the time it wasan impressive achievement (and his proof is very elegant).
No further Fermat primes are known; several have beenfactorised, and several more have been proved compositewithout finding explicit factorisations.
Gauss proved that a regular N-sided polygon can beconstructed with ruler and compasses if and only if N is apower of 2 times a product of distinct Fermat primes.