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A158575
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a(n) = 32*n^2 + 1.
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4
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1, 33, 129, 289, 513, 801, 1153, 1569, 2049, 2593, 3201, 3873, 4609, 5409, 6273, 7201, 8193, 9249, 10369, 11553, 12801, 14113, 15489, 16929, 18433, 20001, 21633, 23329, 25089, 26913, 28801, 30753, 32769, 34849, 36993, 39201, 41473, 43809, 46209, 48673, 51201
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OFFSET
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0,2
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COMMENTS
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The identity (32*n^2+1)^2-(256*n^2+16)*(2*n)^2 = 1 can be written as a(n)^2-A158574(n)*A005843(n)^2 = 1. - Comment rewritten by R. J. Mathar, Oct 16 2009
Sequence found by reading the line segment from 1 to 33 together with the line from 33, in the direction 33, 129, ..., in the square spiral whose vertices are the generalized 18-gonal numbers A274979. - Omar E. Pol, Apr 21 2021
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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.: (1+30*x+33*x^2)/(1-x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
Sum_{n>=0} 1/a(n) = (1 + coth(Pi/(4*sqrt(2)))*Pi/(4*sqrt(2)))/2.
Sum_{n>=0} (-1)^n/a(n) = (1 + cosech(Pi/(4*sqrt(2)))*Pi/(4*sqrt(2)))/2. (End)
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MATHEMATICA
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PROG
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(Magma) I:=[1, 33, 129]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+1*Self(n-3): n in [1..50]]; // Vincenzo Librandi, Feb 15 2012
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CROSSREFS
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Cf. A274979 (generalized 18-gonal numbers).
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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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STATUS
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approved
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