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A136336
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a(n) = a(n-1) + 4*a(n-2) - 4*a(n-3) for n>3.
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2
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1, 2, 4, 7, 15, 27, 59, 107, 235, 427, 939, 1707, 3755, 6827, 15019, 27307, 60075, 109227, 240299, 436907, 961195, 1747627, 3844779, 6990507, 15379115, 27962027, 61516459, 111848107, 246065835, 447392427, 984263339, 1789569707, 3937053355, 7158278827
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OFFSET
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0,2
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LINKS
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FORMULA
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G.f.: (1 + x - 2*x^2 - x^3) / ((1 - x)*(1 - 2*x)*(1 + 2*x)).
a(n) = (8 + (-2)^n + 21*2^n) / 24 for n>0.
(End)
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MATHEMATICA
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LinearRecurrence[{1, 4, -4}, {1, 2, 4, 7}, 50] (* Harvey P. Dale, Jan 14 2019 *)
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PROG
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(PARI) Vec((1 + x - 2*x^2 - x^3) / ((1 - x)*(1 - 2*x)*(1 + 2*x)) + O(x^50)) \\ Colin Barker, Mar 17 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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STATUS
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approved
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