23

This number is a prime.

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There are 23 known natural numbers n such that 2ⁿ does not contain the digit 2. Their sum ends with 23, which is one of them. The prime cases are easy to remember: 2, 3, 23 and ... the 23th prime! [Capelle]

Submitted: 2009-06-10 15:06:38; Last Modified: 2011-02-20 18:33:07.