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EXAMPLE
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a(19) = 8 because there are 8 9-element subsets of {1, 2, ..., 19} having pairwise coprime elements: {1,3,4,5,7,11,13,17,19}, {1,3,5,7,8,11,13,17,19}, {1,3,5,7,11,13,16,17,19}, {1,2,3,5,7,11,13,17,19}, {1,4,5,7,9,11,13,17,19}, {1,2,5,7,9,11,13,17,19}, {1,5,7,8,9,11,13,17,19}, {1,5,7,9,11,13,16,17,19}.
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