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A155212
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a(n) = (n^2 + 9*n + 4)/2.
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5
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2, 7, 13, 20, 28, 37, 47, 58, 70, 83, 97, 112, 128, 145, 163, 182, 202, 223, 245, 268, 292, 317, 343, 370, 398, 427, 457, 488, 520, 553, 587, 622, 658, 695, 733, 772, 812, 853, 895, 938, 982, 1027, 1073, 1120, 1168, 1217, 1267, 1318, 1370, 1423, 1477, 1532
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OFFSET
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0,1
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LINKS
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FORMULA
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G.f.: ( -2 - x + 2*x^2 )/(x - 1)^3 . - R. J. Mathar, Mar 23 2011
Sum_{n>=0} 1/a(n) = 271/280 + 2*Pi*tan(sqrt(65)*Pi/2)/sqrt(65). - Amiram Eldar, Dec 13 2022
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MATHEMATICA
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LinearRecurrence[{3, -3, 1}, {2, 7, 13}, 60] (* Harvey P. Dale, Aug 11 2014 *)
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PROG
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(Magma) I:=[2, 7, 13]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+1*Self(n-3): n in [1..40]]; // Vincenzo Librandi, Feb 26 2012
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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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