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A204465
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Number of n-element subsets that can be chosen from {1,2,...,9*n} having element sum n*(9*n+1)/2.
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1
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1, 1, 9, 85, 1143, 17053, 276373, 4721127, 83916031, 1537408202, 28851490163, 552095787772, 10736758952835, 211657839534446, 4221164530621965, 85031286025167082, 1727896040082882283, 35382865902724442331, 729502230296220422918, 15132164184348997874504
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OFFSET
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0,3
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COMMENTS
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a(n) is the number of partitions of n*(9*n+1)/2 into n distinct parts <=9*n.
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LINKS
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EXAMPLE
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a(2) = 9 because there are 9 2-element subsets that can be chosen from {1,2,...,18} having element sum 19: {1,18}, {2,17}, {3,16}, {4,15}, {5,14}, {6,13}, {7,12}, {8,11}, {9,10}.
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MAPLE
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b:= proc(n, i, t) option remember;
`if`(i<t or n<t*(t+1)/2 or n>t*(2*i-t+1)/2, 0,
`if`(n=0, 1, b(n, i-1, t) +`if`(n<i, 0, b(n-i, i-1, t-1))))
end:
a:= n-> b(n*(9*n+1)/2, 9*n, n):
seq(a(n), n=0..20);
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MATHEMATICA
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b[n_, i_, t_] /; i<t || n<t(t+1)/2 || n>t(2i-t+1)/2 = 0; b[0, _, _] = 1;
b[n_, i_, t_] := b[n, i, t] = b[n, i-1, t] + If[n<i, 0, b[n-i, i-1, t-1]];
a[n_] := b[n(9n+1)/2, 9n, n];
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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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