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A154021
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a(n+2) = 16*a(n+1) - a(n), with a(1)=0, a(2)=4.
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7
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0, 4, 64, 1020, 16256, 259076, 4128960, 65804284, 1048739584, 16714029060, 266375725376, 4245297576956, 67658385505920, 1078288870517764, 17184963542778304, 273881127813935100, 4364913081480183296
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OFFSET
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1,2
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COMMENTS
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If a(n)=x and a(n+1)=y, then 16=(x^2+y^2)/(xy+1).
In general, the sequence a(1)=0, a(2)=U; a(n+2)=U^2*a(n+1)-a(n) has the property that "If a(n)=x and a(n+1)=y then (x^2+y^2)/(xy+1)=U^2".
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LINKS
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FORMULA
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G.f.: 4*x^2/(1 -16*x +x^2).
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MATHEMATICA
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Nest[Append[#, 16Last[#]-#[[-2]]]&, {0, 4}, 20] (* or *) Rest[CoefficientList[Series[4x^2/(1-16x+x^2), {x, 0, 22}], x]] (* Harvey P. Dale, Apr 17 2011 *)
LinearRecurrence[{16, -1}, {0, 4}, 20] (* T. D. Noe, Apr 17 2011 *)
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PROG
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(Magma) I:=[0, 4]; [n le 2 select I[n] else 16*Self(n-1)-Self(n-2): n in [1..30]]; // Vincenzo Librandi, Feb 25 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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375725376 replaced by 266375725376 - R. J. Mathar, Jan 07 2009
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STATUS
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approved
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