That's the divisibility check for 3, yes, but I am pretty sure fdej didn't write that number out in decimal. The proof doesn't work with exponents. You'd have t

That's the divisibility check for 3, yes, but I am pretty sure fdej didn't write that number out in decimal. The proof doesn't work with exponents. You'd have to represent the number in base 10 for digit summing to work. To check, 2^6-1=63 is divisible by three (it might work), but 2^9-1 has a sum of 12 and a value of 511, which is not divisible by three (it's 170 * 3 = 510 + 1).