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A116646
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Number of doubletons in all partitions of n. By a doubleton in a partition we mean an occurrence of a part exactly twice (the partition [4,(3,3),2,2,2,(1,1)] has two doubletons, shown between parentheses).
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4
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0, 0, 1, 0, 2, 2, 4, 5, 10, 11, 20, 25, 38, 49, 73, 91, 131, 167, 228, 291, 392, 493, 653, 822, 1065, 1336, 1714, 2131, 2706, 3354, 4209, 5193, 6471, 7934, 9817, 11990, 14725, 17909, 21875, 26477, 32172, 38797, 46893, 56339, 67804, 81147, 97260, 116017
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,5
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COMMENTS
| a(n) = Sum(k*A116644(n,k), k>=0).
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LINKS
| Alois P. Heinz, Table of n, a(n) for n = 0..1000
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FORMULA
| G.f.=x^2/[(1+x)(1-x^3)product(1-x^j, j=1..infinity)].
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EXAMPLE
| a(6) = 4 because in the partitions of 6, namely [6],[5,1],[4,2],[4,(1,1)],[(3,3)],[3,2,1],[3,1,1,1],[2,2,2],[(2,2),(1,1)],[2,1,1,1,1] and [1,1,1,1,1,1], we have a total of 4 doubletons (shown between parentheses).
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MAPLE
| f:=x^2/(1+x)/(1-x^3)/product(1-x^j, j=1..70): fser:=series(f, x=0, 70): seq(coeff(fser, x, n), n=0..55);
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CROSSREFS
| Cf. A116644. Column k=2 of A197126.
Sequence in context: A116651 A135586 A168542 * A091188 A147678 A195865
Adjacent sequences: A116643 A116644 A116645 * A116647 A116648 A116649
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KEYWORD
| nonn
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AUTHOR
| Emeric Deutsch (deutsch(AT)duke.poly.edu), Feb 20 2006
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