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A180282
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Number of arrangements of n indistinguishable balls in n boxes with the maximum number of balls in any box equal to 2.
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2
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2, 6, 18, 50, 140, 392, 1106, 3138, 8952, 25652, 73788, 212940, 616226, 1787606, 5196626, 15134930, 44152808, 128996852, 377379368, 1105350728, 3241135526, 9513228122, 27948336380, 82176836300, 241813226150, 712070156202, 2098240353906, 6186675630818
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OFFSET
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2,1
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LINKS
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FORMULA
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a(n) = Sum_{j=1..n} binomial(n,j)*binomial(n-j,j) = 2*A097861(n).
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MAPLE
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b:= proc(n, i, k) option remember; `if`(n=0, 1,
`if`(i=0, 0, add(b(n-j, i-1, k), j=0..min(n, k))))
end:
a:= n-> (k-> b(n$2, k)-b(n$2, k-1))(2):
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PROG
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(PARI) for(n=2, 29, print1(sum(j=1, n, binomial(n, j)*binomial(n-j, j)), ", ")) \\ Hugo Pfoertner, Dec 13 2019
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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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