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A158657
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a(n) = 784*n^2 - 28.
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2
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756, 3108, 7028, 12516, 19572, 28196, 38388, 50148, 63476, 78372, 94836, 112868, 132468, 153636, 176372, 200676, 226548, 253988, 282996, 313572, 345716, 379428, 414708, 451556, 489972, 529956, 571508, 614628, 659316, 705572, 753396, 802788, 853748, 906276, 960372
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OFFSET
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1,1
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COMMENTS
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The identity (56*n^2 - 1)^2 - (784*n^2 - 28)*(2*n)^2 = 1 can be written as A158658(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.: 28*x*(-27 - 30*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/(2*sqrt(7)))*Pi/(2*sqrt(7)))/56.
Sum_{n>=1} (-1)^(n+1)/a(n) = (cosec(Pi/(2*sqrt(7)))*Pi/(2*sqrt(7)) - 1)/56. (End)
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MATHEMATICA
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LinearRecurrence[{3, -3, 1}, {756, 3108, 7028}, 50] (* Vincenzo Librandi, Feb 17 2012 *)
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PROG
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(Magma) I:=[756, 3108, 7028]; [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 17 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 rephrased and redundant formula replaced by R. J. Mathar, Oct 19 2009
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STATUS
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approved
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