Divisor of Fermat Number
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Theorem
Let $F_n$ be a Fermat number.
Let $m$ be divisor of $F_n$.
Result by Euler
Then $m$ is in the form:
- $k \, 2^{n + 1} + 1$
where $k \in \Z_{>0}$ is an integer.
Refinement by Lucas
Let $n \ge 2$.
Then $m$ is in the form:
- $k \, 2^{n + 2} + 1$
Historical Note
In $1747$, Leonhard Paul Euler proved that a divisor of a Fermat number $F_n$ is always in the form $k \, 2^{n + 1} + 1$.
This was later refined to $k \, 2^{n + 2} + 1$ by François Édouard Anatole Lucas.