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A085789
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Partial sums of n 3-spaced triangular numbers beginning with t(2), e.g., a(2) = t(2) + t(5) = 3 + 15 = 18.
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2
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3, 18, 54, 120, 225, 378, 588, 864, 1215, 1650, 2178, 2808, 3549, 4410, 5400, 6528, 7803, 9234, 10830, 12600, 14553, 16698, 19044, 21600, 24375, 27378, 30618, 34104, 37845, 41850, 46128, 50688, 55539, 60690, 66150, 71928, 78033, 84474, 91260, 98400, 105903
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OFFSET
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1,1
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COMMENTS
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Sums of rows of triangle A100345 (n>0).
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LINKS
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FORMULA
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a(n) = 3/2 * n^2*(n+1).
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MATHEMATICA
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CoefficientList[Series[3 (1 + 2 x) / (1 - x)^4, {x, 0, 40}], x](* Vincenzo Librandi, Aug 14 2017 *)
LinearRecurrence[{4, -6, 4, -1}, {3, 18, 54, 120}, 50] (* Harvey P. Dale, May 14 2023 *)
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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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