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A151974
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a(n) = n*(n+1)*(n+2)*(n+3)*(n+4)/8.
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4
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0, 15, 90, 315, 840, 1890, 3780, 6930, 11880, 19305, 30030, 45045, 65520, 92820, 128520, 174420, 232560, 305235, 395010, 504735, 637560, 796950, 986700, 1210950, 1474200, 1781325, 2137590, 2548665, 3020640, 3560040, 4173840, 4869480, 5654880, 6538455, 7529130
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OFFSET
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0,2
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COMMENTS
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Also the number of 4-cycles in the (n+3)-triangular graph. - Eric W. Weisstein, Aug 14 2017
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LINKS
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FORMULA
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a(n) = n*(n+1)*(n+2)*(n+3)*(n+4)/8.
a(n) = 6*a(n-1) - 15*a(n-2) + 20*a(n-3) - 15*a(n-4) + 6*a(n-5) - a(n-6). - Eric W. Weisstein, Aug 14 2017
Sum_{n>=1} 1/a(n) = 1/12.
Sum_{n>=1} (-1)^(n+1)/a(n) = 16*log(2)/3 - 131/36. (End)
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MAPLE
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MATHEMATICA
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Table[Pochhammer[n, 5]/8, {n, 0, 31}] (* or *)
Rest @ CoefficientList[Series[15 x^2/(1 - x)^6, {x, 0, 32}], x] (* Michael De Vlieger, Feb 12 2017 *)
LinearRecurrence[{6, -15, 20, -15, 6, -1}, {0, 15, 90, 315, 840, 1890}, 20] (* Eric W. Weisstein, Aug 14 2017 *)
Table[(24 n+50 n^2+35 n^3+10 n^4+n^5)/8, {n, 0, 40}] (* or *) Table[Times@@Range[n, n+4]/8, {n, 0, 40}] (* Harvey P. Dale, Mar 06 2024 *)
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PROG
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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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EXTENSIONS
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STATUS
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approved
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