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Repunits and bases (Posted on 2013-07-24) Difficulty: 3 of 5
112=3 and 1113=13 are prime. However, 11114=85=5*17 and 111115=781=11*71 are not prime. What is the least number n>3 such that the repunit consisting of n 1's in base n is prime?

See The Solution Submitted by Math Man    
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Question re: Answer: | Comment 3 of 6 |
(In reply to Answer: by broll)

Why is it obvious that x is prime only if n is prime?
  Posted by Charlie on 2013-07-24 12:42:29

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