  Let n(2,k) denote the largest integer n for which there exists a set A of k
nonnegative integers such that the sumset 2A contains {0,1,2,...,n-1}. A
classical problem in additive number theory is to find an upper bound for
n(2,k). In this paper it is proved that limsup_{k\to\infty} n(2,k)/k^2 \leq
0.4789.
