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A152759
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3 times 9-gonal (or nonagonal) numbers: 3n(7n-5)/2.
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13
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0, 3, 27, 72, 138, 225, 333, 462, 612, 783, 975, 1188, 1422, 1677, 1953, 2250, 2568, 2907, 3267, 3648, 4050, 4473, 4917, 5382, 5868, 6375, 6903, 7452, 8022, 8613, 9225, 9858, 10512, 11187, 11883, 12600, 13338, 14097, 14877, 15678
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = (21n^2 - 15n)/2 = A001106(n)*3.
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MATHEMATICA
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CoefficientList[Series[3 x (1 + 6 x) / (1 - x)^3, {x, 0, 60}], x] (* Vincenzo Librandi, Jun 05 2013 *)
LinearRecurrence[{3, -3, 1}, {0, 3, 27}, 40] (* Harvey P. Dale, May 26 2015 *)
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PROG
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CROSSREFS
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Cf. numbers of the form n*(n*k-k+6))/2, this sequence is the case k=21: see Comments lines of A226492.
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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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