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A158738
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a(n) = 72*n^2 - 1.
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2
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71, 287, 647, 1151, 1799, 2591, 3527, 4607, 5831, 7199, 8711, 10367, 12167, 14111, 16199, 18431, 20807, 23327, 25991, 28799, 31751, 34847, 38087, 41471, 44999, 48671, 52487, 56447, 60551, 64799, 69191, 73727, 78407, 83231, 88199, 93311, 98567, 103967, 109511, 115199
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OFFSET
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1,1
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COMMENTS
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The identity (72*n^2 - 1)^2 - (1296*n^2 - 36)*(2*n)^2 = 1 can be written as a(n)^2 - A158737(n)*A005843(n)^2 = 1.
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LINKS
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Vincenzo Librandi, X^2-AY^2=1, Math Forum, 2007. [Wayback Machine link]
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FORMULA
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G.f.: x*(-71 - 74*x + x^2)/(x-1)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
Sum_{n>=1} 1/a(n) = (1 - cot(Pi/(6*sqrt(2)))*Pi/(6*sqrt(2)))/2.
Sum_{n>=1} (-1)^(n+1)/a(n) = (cosec(Pi/(6*sqrt(2)))*Pi/(6*sqrt(2)) - 1)/2. (End)
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MAPLE
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MATHEMATICA
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72Range[40]^2-1 (* or *) LinearRecurrence[{3, -3, 1}, {71, 287, 647}, 40] (* Harvey P. Dale, May 01 2011 *)
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PROG
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(Magma) I:=[71, 287, 647]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+1*Self(n-3): n in [1..40]]; // Vincenzo Librandi, Feb 21 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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EXTENSIONS
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Comment rewritten and formula replaced by R. J. Mathar, Oct 22 2009
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STATUS
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approved
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