Composite Fermat number

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Definition

A composite Fermat number is a Fermat number that is also a composite number, i.e., it is not a Fermat prime.

Facts

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 .

Relation with other properties

Weaker properties