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Search: id:A131661
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| A131661 |
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Number of compositions of n such that the cardinality of the set of parts is 2. |
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+0
1
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| 0, 0, 2, 5, 14, 22, 44, 68, 107, 172, 261, 396, 606, 950, 1414, 2238, 3418, 5411, 8368, 13297, 20840, 33268, 52549, 84120, 133775, 214611, 343025, 551064, 883600, 1421767, 2284870, 3680296, 5924725, 9551161, 15393855, 24834827, 40061700
(list; graph; listen)
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OFFSET
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1,3
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FORMULA
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G.f.: Sum(Sum(x^(i+j)*(x^i+x^j-2)/((x^i-1)*(x^j-1)*(x^i+x^j-1)),j=1..i-1),i=2..infinity).
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MATHEMATICA
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Rest@ CoefficientList[ Series[ Sum[ x^(i + j)*(x^i + x^j - 2)/((x^i - 1)*(x^j - 1)*(x^i + x^j - 1)), {i, 2, 37}, {j, i - 1}], {x, 0, 37}], x] - Robert G. Wilson v (rgwv(AT)rgwv.com), Sep 16 2007
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CROSSREFS
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Cf. A000005, A002133, A005772, A088142.
Sequence in context: A089410 A118670 A015633 this_sequence A076664 A049939 A133375
Adjacent sequences: A131658 A131659 A131660 this_sequence A131662 A131663 A131664
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KEYWORD
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easy,nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)eunet.rs), Sep 13 2007
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EXTENSIONS
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More terms from Robert G. Wilson v (rgwv(AT)rgwv.com), Sep 16 2007
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