5

This number is a prime.

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5 = 12 in base 3. There is no other prime p that can be expressed as p = 12...n in base n+1, because p has n as a factor if n is odd, or n/2, if n is even. [Necula]

The only single-digit prime with a prime ASCII code: 35 (in base 16) = 53. [Necula]