A sum-free set S is a set for which the intersection of S and the sumset S + S is empty. For example, the sum-free sets of {1, 2, 3} are ∅, {1}, {2}, {3}, {1, 3}, and {2, 3}. The numbers of sum-free subsets of {1, 2, ..., n} for n = 0, 1, ... are 1, 2, 3, 6, 9, 16, 24, 42, 61, ... (OEIS A007865).