Fermat number
If 2"k" + 1 is prime and "k" &gt; 0, then "k" must be a power of 2, so 2"k" + 1 is a Fermat number; such primes are called Fermat primes. As of 2021, the only known Fermat primes are "F"0 = 3, "F"1 = 5, "F"2 = 17, "F"3 = 257, and "F"4 = 65537 ; heuristics suggest that there are no more.