23
This number is a prime.
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23 is the only known number p where Rp is a repunit prime, 10^p-p is prime, and p is a prime factor of a Mersenne number with repunit exponent. I.e., 23 is factor of 2^11-1. [Luhn]
Submitted: 2004-09-08 19:50:46; Last Modified: 2012-06-06 17:08:05.
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