511

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Number

$511$ (five hundred and eleven) is:

$7 \times 73$

Historical Note

$511$ 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 $2^9 - 1 = 511 = 7 \times 73$.