A repunit is a number consisting solely of ones (such as 11 or 11111).
Let us call p(n) a 10-base integer represented by a string of n ones, e.g. p(1)=1, p(5)=11111 etc.
Most of the repunit numbers are composite.
2, 19,23,317 are the first four indices of prime repunits.

Prove: For a prime repunit p(n) to be prime, n has to be prime.

(In reply to re(2): Another proof by Gamer)

yOUR PROOF IS FLAWLESS ,I GOOFED,  S O R R Y 

Posted by Ady TZIDON on 2011-01-14 02:40:36