Composite Fermat number
A composite Fermat number is a Fermat number that is also a composite number, i.e., it is not a Fermat prime.
Constraints on prime divisors
Further information: Prime divisor of Fermat number is congruent to one modulo large power of two
If , and is a composite Fermat number, then divides for any prime divisor of . As a corollary, divides for any divisor of .