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EXAMPLE
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a(16) = 8 because there are 8 7-element subsets of {1, 2, ..., 16} having pairwise coprime elements: {1,5,7,9,11,13,16}, {1,5,7,8,9,11,13}, {1,2,3,5,7,11,13}, {1,2,5,7,9,11,13}, {1,3,4,5,7,11,13}, {1,3,5,7,8,11,13}, {1,3,5,7,11,13,16}, {1,4,5,7,9,11,13}.
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