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A158590
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a(n) = 324*n^2 + 18.
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2
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18, 342, 1314, 2934, 5202, 8118, 11682, 15894, 20754, 26262, 32418, 39222, 46674, 54774, 63522, 72918, 82962, 93654, 104994, 116982, 129618, 142902, 156834, 171414, 186642, 202518, 219042, 236214, 254034, 272502, 291618, 311382, 331794, 352854, 374562, 396918
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OFFSET
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0,1
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COMMENTS
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The identity (36*n^2 + 1)^2 - (324*n^2 + 18)*(2*n)^2 = 1 can be written as A158591(n)^2 - a(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.: -18*(1 + 16*x + 19*x^2)/(x-1)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
Sum_{n>=0} 1/a(n) = (coth(Pi/(3*sqrt(2)))*Pi/(3*sqrt(2)) + 1)/36.
Sum_{n>=0} (-1)^n/a(n) = (cosech(Pi/(3*sqrt(2)))*Pi/(3*sqrt(2)) + 1)/36. (End)
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MATHEMATICA
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LinearRecurrence[{3, -3, 1}, {18, 342, 1314}, 50] (* Vincenzo Librandi, Feb 16 2012 *)
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PROG
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(Magma) I:=[18, 342, 1314]; [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 16 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, formula replaced by R. J. Mathar, Oct 28 2009
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STATUS
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approved
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