3

This number is a prime.

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Jason Funk © 2002

3 is the only positive integer n which is the difference between the least prime greater than n and the greatest prime less than n. [Hartley]

As at February 2003, 3 and 13 are the only known prime numbers n for which the number of n-ominoes is also prime. [Hartley]

3 and 103 are prime, as are 3*2^103-1 and 103*2^3-1. There is no other pair of primes less than 1000 with this property. [Hartley]