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A048000
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Number of nonempty subsets of {1,2,...,n} in which exactly 4/5 of the elements are <= n/3.
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1
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 9, 10, 50, 55, 60, 180, 195, 210, 490, 525, 560, 1240, 1326, 1413, 3645, 3933, 4230, 12750, 13860, 15015, 45375, 49335, 53460, 150524, 163175, 176345, 470665, 509067, 549094, 1461278, 1580761
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OFFSET
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1,12
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LINKS
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FORMULA
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a(n) = Sum_{k=1..floor(n/5)} binomial(floor(n/3), 4*k)*binomial(ceiling(2*n/3), k). - Robert Israel, Feb 05 2017
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MAPLE
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f:= proc(n) local k;
add(binomial(floor(n/3), 4*k/5)*binomial(n-floor(n/3), k/5), k=5..n, 5)
end proc:
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PROG
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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