511/Historical Note
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Historical Note on $511$
$511$ was the $1$st number of the form $2^n - 1$, where $n$ is odd, that was demonstrated to be composite.
This was done by Hudalrichus Regius in $1636$, who demonstrated that $2^9 - 1 = 511 = 7 \times 73$.
Sources
- 1919: Leonard Eugene Dickson: History of the Theory of Numbers: Volume $\text { I }$ ... (previous) ... (next): Preface