(1) Find a test for divisibility by 111.

(2) Extending this, find a test for divisibility by any number consisting solely of 1's.

Please explain why your tests work.

(In reply to re: Divide and conquer (spoiler) & a challenge by Kenny M)

I think Steve's is a generalization of the standard divisibility test for 11, but instead of odd and even positions, you have positions mod 3, or in general mod the number of 1's in the potential divisor.  The odd-even test seems to be the best one available for 11, as that's the one always used.

Posted by Charlie on 2013-12-15 10:14:07