A repunit is a number consisting solely of ones (such as 11
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
(In reply to re(2): proof
Nothing wrong with your proof, I agree with all your statements, as the Bard said..."to the selfsame tune and words".
Whatever I added was a remark , to distinguishb between necessary and sufficent- with no claim re anything you wrote.
To sum uo: conrapositives are of equal weight.