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1, 125, 2197, 68921, 1771561, 48627125, 1305751357, 35319837041, 953054410321, 25737699078125, 694870802988517, 18761935323400361, 506568440928284281, 13677382220238009125, 369289011109685057677, 9970806079491650694881, 269211739130501631841441
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OFFSET
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1,2
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LINKS
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FORMULA
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G.f.: x*(1 + 105*x - 513*x^2 - 729*x^3) / ((1 + x)*(1 - 3*x)*(1 + 9*x)*(1 - 27*x)). - R. J. Mathar, Sep 09 2008
a(n) = ((-1)^n + 3^(1+n) + (-1)^n*3^(1+2*n) + 27^n) / 8 for n>0. - Colin Barker, Dec 23 2017
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EXAMPLE
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a(3) = 2197 = 13^3 = (A046717(a))^3.
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MATHEMATICA
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Rest@ Nest[Append[#, 2 #[[-1]] + 3 #[[-2]]] &, {1, 1}, 15]^3 (* or *)
Rest@ CoefficientList[Series[x (1 + 105 x - 513 x^2 - 729 x^3)/((1 + 9 x) (1 - 3 x) (1 - 27 x) (1 + x)), {x, 0, 16}], x] (* Michael De Vlieger, Dec 22 2017 *)
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PROG
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(PARI) Vec(x*(1 + 105*x - 513*x^2 - 729*x^3) / ((1 + x)*(1 - 3*x)*(1 + 9*x)*(1 - 27*x)) + O(x^40)) \\ Colin Barker, Dec 23 2017
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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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STATUS
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approved
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