For a triple (n,t,m) of positive integers, we attach to each t-subset S={a 1 ,⋯,a t }⊆{1,⋯,n} the sum f(S)=a 1 +⋯+a t (modulo m). We ask: for which triple (n,t,m) are the nt values of f(S) uniformly distributed in the residue classes modm? The obvious necessary condition, that m divides nt, is not sufficient, but a q-analogue of that condition is both necessary and sufficient, namely: q m -1
... [Show full abstract] q-1dividestheGaussianpolynomialnt q · We show that this condition is equivalent to: for each divisor d>1 of m, we have tmodd>nmodd. Two proofs are given, one by generating functions and another via a bijection. We study the analogous question on the full power set of [n]: given (n,m); when are the 2 n subset sums modulo m equidistributed into the residue classes? Finally, we obtain some asymptotic information about the distribution when it is not uniform, and discuss some open questions.