Past exam of the options II course of the University of Oxford / 2010 / C / C2 / v / Solution

Part (ii), with exponent $30$, shows that a prime $q$ fails to divide the sum precisely when $q-1$ divides $30$. The positive divisors are $1,2,3,5,6,10,15,30$. Adding one gives $2,3,4,6,7,11,16,31$, of which the primes areFor these primes the sum is $-1$ modulo $q$; for every other prime it is zero.