8191 is a more-than-two-digit repunit prime in two bases:
8191 = 111 in base 90 = 1111111111111 in base 2.
Find the only other number with this property.
it turns out the proof that this are in fact the only two such numbers is as yet incomplete, at least according to this wiki and a google search on the conjecture did not turn up any more recent work other than expanding the search bounds.
Posted by Daniel
on 2018-07-27 07:19:07