Question

You won’t need quadratic reciprocity. The number a is divisible by 11 if and only if the alternating sum of the digits of a is divisible by 11. That is, if the digits of a are a1a2 a3 ...ad−1 ad, the alternating sum means to take a1 −a2 +a3 −a4 ..., with alternating plus and minus signs. Write a as a sum of multiples of powers of 10 and reduce modulo 11.

Answer 1