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A001513
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a(n) = (6*n+1)*(6*n+5).
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2
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5, 77, 221, 437, 725, 1085, 1517, 2021, 2597, 3245, 3965, 4757, 5621, 6557, 7565, 8645, 9797, 11021, 12317, 13685, 15125, 16637, 18221, 19877, 21605, 23405, 25277, 27221, 29237, 31325, 33485, 35717, 38021, 40397, 42845, 45365, 47957, 50621, 53357, 56165, 59045
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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.: (-5 - 62*x - 5*x^2) / (x-1)^3. - R. J. Mathar, Jan 19 2013
Sum_{n>=0} (-1)^n/a(n) = log(2+sqrt(3))/(4*sqrt(3)).
Product_{n>=0} (1 - 1/a(n)) = 2*cos(sqrt(5)*Pi/6).
Product_{n>=0} (1 + 1/a(n)) = 2*cos(sqrt(3)*Pi/6). (End)
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MATHEMATICA
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a[n_] := (6*n + 1)*(6*n + 5); Array[a, 40, 0] (* Amiram Eldar, Feb 19 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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STATUS
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approved
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