Abstract
Let A be a finite nonempty subset of an additive abelian group G, and let Sigma(A) denote the set of all group elements representable as a sum of some subset of A. We prove that |Sigma(A)| >= |H| + 1/64 |A \ H|^2 where H is the stabilizer of Sigma(A).
Our result implies that \Sigma(A) = Z/nZ for every set A of units of Z/nZ with |A| \ge 8 \sqrt{n}. This consequence was first proved by Erd\H{o}s and Heilbronn for n prime, and by Vu (with a weaker constant) for general n.