Definition:Fermat Number

Definition
A Fermat number is a natural number of the form $2^{\paren {2^n} } + 1$, where $n = 0, 1, 2, \ldots$.

The number $2^{\paren {2^n} } + 1$ is, in this context, often denoted $F_n$.

Also see

 * Definition:Fermat Prime
 * Fermat Prime Conjecture