1023
This number is a composite.
The smallest example of M(x), i.e., a Mersenne number, where M(x) = 3 * M(x/2) * p, where M(x/2) and p are prime. This is
M(10) = 3 * M(5) * p, or 1023 = 3 * 31 * 11. [Ganfield]
(There are 2 curios for this number that have not yet been approved by an editor.)
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