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A087124
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a(n) = Fibonacci(n) + Fibonacci(2n+1).
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3
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1, 3, 6, 15, 37, 94, 241, 623, 1618, 4215, 11001, 28746, 75169, 196651, 514606, 1346879, 3525565, 9229062, 24160401, 63250167, 165586906, 433505383, 1134920881, 2971243730, 7778788417, 20365086099, 53316412566, 139584058863
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OFFSET
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0,2
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COMMENTS
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For n>=1, a(n) is the coefficient of x in the reduction by x^2->x+1 of the polynomial 1+x^n+x^(2n+1). For an introduction to reductions of polynomials by substitutions such as x^2->x+1, see A192232. - Clark Kimberling, Jul 01 2011
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LINKS
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FORMULA
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G.f.: (1-2*x)*(1+x-x^2)/((1-3*x+x^2)*(1-x-x^2)). - Colin Barker, Mar 12 2012
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MATHEMATICA
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CoefficientList[Series[(1-2*x)*(1+x-x^2)/((1-3*x+x^2)*(1-x-x^2)), {x, 0, 1001}], x] (* Vincenzo Librandi, Mar 13 2012 *)
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PROG
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(Magma) [Fibonacci(n)+Fibonacci(2*n+1): n in [0..40]]; // Vincenzo Librandi, Mar 13 2012
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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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