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A047168
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Number of nonempty subsets of {1,2,...,n} in which exactly 4/5 of the elements are <= n/2.
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2
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0, 0, 0, 0, 0, 0, 0, 4, 5, 25, 30, 90, 105, 245, 280, 588, 666, 1458, 1665, 4125, 4785, 12705, 14850, 38830, 45331, 113399, 131859, 320411, 371735, 903175, 1048840, 2594540, 3021240, 7594920, 8863698, 22366458, 26122302, 65579982, 76575225, 191126529
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OFFSET
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1,8
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LINKS
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FORMULA
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a(n) = Sum_{k>=1} binomial(floor(n/2), 4*k)*binomial(ceiling(n/2), k). - Andrew Howroyd, Apr 11 2021
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PROG
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(PARI) a(n) = {my(m=n\2); sum(k=1, m\4, binomial(m, 4*k)*binomial(n-m, k))} \\ Andrew Howroyd, Apr 11 2021
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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