 Initially, with formula_8, there is only one pairwise sum, 1 + 1 = 2.The next term in the sequence, formula_9, is 2 since the pairwise sums then are 2, 3 and 4, i.e., they are distinct.Then, formula_10 can't be 3 because there would be the non-distinct pairwise sums 1 + 3 = 2 + 2 = 4.