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A081270
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Diagonal of triangular spiral in A051682.
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3
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3, 16, 38, 69, 109, 158, 216, 283, 359, 444, 538, 641, 753, 874, 1004, 1143, 1291, 1448, 1614, 1789, 1973, 2166, 2368, 2579, 2799, 3028, 3266, 3513, 3769, 4034, 4308, 4591, 4883, 5184, 5494, 5813, 6141, 6478, 6824, 7179, 7543, 7916, 8298, 8689, 9089, 9498
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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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a(n) = 3C(n, 0)+13C(n, 1)+9C(n, 2); binomial transform of (3, 13, 9, 0, 0, 0, ...).
a(n) = (9n^2+17n+6)/2.
G.f.: (3+7x-x^2)/(1-x)^3.
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MATHEMATICA
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CoefficientList[Series[(3+7x-x^2)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 08 2012 *)
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PROG
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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