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A273220
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a(n) = 8n^2 - 12n + 1.
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1
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9, 37, 81, 141, 217, 309, 417, 541, 681, 837, 1009, 1197, 1401, 1621, 1857, 2109, 2377, 2661, 2961, 3277, 3609, 3957, 4321, 4701, 5097, 5509, 5937, 6381, 6841, 7317, 7809, 8317, 8841, 9381, 9937, 10509, 11097, 11701, 12321, 12957, 13609, 14277, 14961, 15661
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OFFSET
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2,1
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COMMENTS
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Sequence may be obtained by starting with the segment (9, 37) followed by the line from 37 in the direction 37, 81,... in the square spiral whose vertices are the generalized hexagonal numbers (A000217). - Omar E. Pol, Jun 26 2016
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LINKS
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FORMULA
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a(n) = 3*a(n-1)-3*a(n-2)+a(n-3) for n>4.
G.f.: x^2*(9+10*x-3*x^2) / (1-x)^3.
(End)
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MATHEMATICA
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Table[8 n^2 - 12 n + 1, {n, 2, 45}] (* or *)
Drop[#, 2] &@ CoefficientList[Series[x^2 (9 + 10 x - 3 x^2)/(1 - x)^3, {x, 0, 45}], x] (* Michael De Vlieger, Jun 26 2016 *)
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PROG
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(PARI) Vec(x^2*(9+10*x-3*x^2)/(1-x)^3 + O(x^50)) \\ Colin Barker, May 18 2016
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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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STATUS
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approved
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