We adapt a number-theoretic technique of Yu to prove a purely analytic theorem: if f∈L1(ℝ)∩L2(ℝ) is nonnegative and supported on an interval of length I, then the supremum of f*f is at least 0.631‖f‖12/I. This improves the previous bound of 0.591389‖f‖12/I. Consequently, we improve the known bounds on several related number-theoretic problems. For a set A⊆{1, 2, …, n}, let g be the maximum multiplicity of any element of the multiset {a1+a2 : ai∈A}. Our main corollary is the inequality gn>0.631|A|2, which holds uniformly for all g, n, and A.