\begin{definition}[Definition:Fermat Number]
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 $F_n$.
\end{definition}