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A067727
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a(n) = 7*n^2 + 14*n.
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9
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21, 56, 105, 168, 245, 336, 441, 560, 693, 840, 1001, 1176, 1365, 1568, 1785, 2016, 2261, 2520, 2793, 3080, 3381, 3696, 4025, 4368, 4725, 5096, 5481, 5880, 6293, 6720, 7161, 7616, 8085, 8568, 9065, 9576, 10101, 10640, 11193, 11760, 12341, 12936
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OFFSET
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1,1
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COMMENTS
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Positive numbers k such that 7*(7 + k) is a perfect square.
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = 3/28.
Sum_{n>=1} (-1)^(n+1)/a(n) = 1/28. (End)
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MAPLE
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MATHEMATICA
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Select[ Range[15000], IntegerQ[ Sqrt[ 7(7 + # )]] & ]
CoefficientList[Series[7*(3-x)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 08 2012 *)
LinearRecurrence[{3, -3, 1}, {21, 56, 105}, 50] (* Harvey P. Dale, Dec 07 2022 *)
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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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