Post #1098420

@Taneb@hacksrus.xyz … ok, that was badly explained. The _point_ of working mod 9 is that, mod 9, a number is congruent to the sum of its decimal digits (because any power of 10 is congruent to 1), so that normalises out the order that the digits occur in. If you work mod 11 then a number is congruent to the _alternating_ sum of its digits, because 10 is congruent to −1 rather than +1. So it's _not_ as easy like that. It should have been a red flag when I wrote "I've never thought of this before, but …". Sometimes the reason you've never thought of a thing before is that it's nonsense!