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A094013
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Expansion of (1-4*x)/(1-4*x-4*x^2).
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7
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1, 0, 4, 16, 80, 384, 1856, 8960, 43264, 208896, 1008640, 4870144, 23515136, 113541120, 548225024, 2647064576, 12781158400, 61712891904, 297976201216, 1438756372480, 6946930294784, 33542746669056, 161958707855360
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OFFSET
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0,3
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COMMENTS
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a(n) is the irrational part of circle radii in nested circles and squares inspired by Vitruvian Man, starting with a square whose sides are of length 4 (in some units). The radius of the circle is an integer in the real quadratic number field Q(sqrt(2)), namely R(n) = A(n-1) + B(m)*sqrt(2) with A(-1)=1, for n >= 1, A(n-1) = A170931(n-1)*-1^(n-1); and B(n) = A094013(n)*-1^n. See illustrations in the links. - Kival Ngaokrajang, Feb 15 2015
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LINKS
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FORMULA
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a(n) = (2 + 2*sqrt(2))^n*(1/2 - sqrt(2)/4) + (2 - 2*sqrt(2))^n*(1/2 + sqrt(2)/4).
E.g.f.: exp(2*x)*( cosh(2*sqrt(2)*x) - (1/sqrt(2))*sinh(2*sqrt(2)*x) ). (End)
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MATHEMATICA
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CoefficientList[Series[(1-4x)/(1-4x-4x^2), {x, 0, 40}], x] (* or *) LinearRecurrence[{4, 4}, {1, 0}, 40] (* Harvey P. Dale, May 21 2012 *)
Table[2^n*Fibonacci[n-1, 2], {n, 0, 40}] (* G. C. Greubel, Dec 04 2021 *)
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PROG
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(PARI) Vec((1-4*x)/(1-4*x-4*x^2) + O(x^30)) \\ Michel Marcus, Feb 15 2015
(Magma) [n le 2 select 2-n else 4*(Self(n-1) + Self(n-2)): n in [1..41]]; // G. C. Greubel, Dec 04 2021
(Sage) [2^n*lucas_number1(n-1, 2, -1) for n in (0..40)] # G. C. Greubel, Dec 04 2021
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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