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A047170
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Number of nonempty subsets of {1,2,...,n} in which exactly 5/6 of the elements are <= n/2.
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2
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0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 6, 36, 42, 147, 168, 448, 504, 1134, 1260, 2565, 2827, 5687, 6270, 13860, 15444, 39039, 44044, 119119, 135135, 360815, 408968, 1039808, 1174224, 2842604, 3197904, 7515054, 8436114, 19773414, 22203020, 53093125, 59750649, 146794494
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OFFSET
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1,10
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LINKS
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FORMULA
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a(n) = Sum_{k>=1} binomial(floor(n/2), 5*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\5, binomial(m, 5*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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