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A170832
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a(n) = 3^(floor(n/2))+3^(floor(n/2)-1)-3^(floor((n-1)/3)).
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3
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3, 3, 9, 9, 33, 27, 99, 99, 297, 297, 945, 891, 2835, 2835, 8505, 8505, 26001, 25515, 78003, 78003, 234009, 234009, 706401, 702027, 2119203, 2119203, 6357609, 6357609, 19112193, 19072827, 57336579, 57336579, 172009737, 172009737, 516383505, 516029211, 1549150515
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OFFSET
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2,1
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LINKS
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R. P. Stanley, Problem 11348, Amer. Math. Monthly, 117 (2010), 87-88.
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FORMULA
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a(n) = 3*a(n-2) +3*a(n-3) -9*a(n-5). G.f.: -3*x^2*(x^4+3*x^3-x-1) / ((3*x^2-1)*(3*x^3-1)). - Colin Barker, Jul 26 2013
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MATHEMATICA
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LinearRecurrence[{0, 3, 3, 0, -9}, {3, 3, 9, 9, 33}, 40] (* Harvey P. Dale, Dec 18 2015 *)
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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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STATUS
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approved
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