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A027777
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a(n) = 2*(n+1)*binomial(n+2,4).
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3
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6, 40, 150, 420, 980, 2016, 3780, 6600, 10890, 17160, 26026, 38220, 54600, 76160, 104040, 139536, 184110, 239400, 307230, 389620, 488796, 607200, 747500, 912600, 1105650, 1330056, 1589490, 1887900, 2229520, 2618880, 3060816, 3560480, 4123350, 4755240, 5462310
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OFFSET
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2,1
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COMMENTS
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Number of 7-subsequences of [ 1, n ] with just 2 contiguous pairs.
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LINKS
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FORMULA
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G.f.: 2*(3+2x)*x^2/(1-x)^6.
Sum_{n>=2} 1/a(n) = Pi^2 - 29/3.
Sum_{n>=2} (-1)^n/a(n) = Pi^2/2 + 8*log(2) - 31/3. (End)
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MATHEMATICA
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Table[2(n+1)Binomial[n+2, 4], {n, 2, 35}] (* Harvey P. Dale, Feb 03 2011 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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