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?

(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