123

This number is a composite.

+ 123 x 2123 - 1 is prime. [Noll]

+ The number formed by the concatenation of odd numbers from 123 down to 1 is prime. [Patterson]

+ 12342 - 1232 - 456 is prime! [Leal]

(There is one curio for this number that has not yet been approved by an editor.)

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