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A157915
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a(n) = 625*n^2 + 25.
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2
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650, 2525, 5650, 10025, 15650, 22525, 30650, 40025, 50650, 62525, 75650, 90025, 105650, 122525, 140650, 160025, 180650, 202525, 225650, 250025, 275650, 302525, 330650, 360025, 390650, 422525, 455650, 490025, 525650, 562525, 600650, 640025, 680650, 722525, 765650
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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G.f: x*(650 + 575*x + 25*x^2)/(1-x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). (End)
Sum_{n>=1} 1/a(n) = (coth(Pi/5)*Pi/5 - 1)/50.
Sum_{n>=1} (-1)^(n+1)/a(n) = (1 - cosech(Pi/5)*Pi/5)/50. (End)
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MATHEMATICA
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LinearRecurrence[{3, -3, 1}, {650, 2525, 5650}, 40] (* Vincenzo Librandi, Feb 10 2012 *)
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PROG
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(Magma) I:=[650, 2525, 5650]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+Self(n-3): n in [1..50]]; // Vincenzo Librandi, Feb 10 2012
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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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