2

This number is a prime.

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F. Viete (1540-1603) expressed π as an infinite product containing only 2 (and its reciprocal 1/2). [Caldwell]

It is possible to measure all of the integer distances from one to six on a six-inch ruler with just 2 marks. For example, the distance from the 2 to the right end is four inches. [Caldwell]