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A137340
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a(n) = 2*a(n-1) + 3*a(n-2), with a(0) = 1, a(1) = 9.
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1
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1, 9, 21, 69, 201, 609, 1821, 5469, 16401, 49209, 147621, 442869, 1328601, 3985809, 11957421, 35872269, 107616801, 322850409, 968551221, 2905653669, 8716961001, 26150883009, 78452649021, 235357947069, 706073841201, 2118221523609, 6354664570821, 19063993712469
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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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a(n) = (5*3^n - 3*(-1)^n)/2.
a(2*n) = (5*9^n - 3)/2.
a(2*n+1) = (15*9^n + 3)/2.
a(n) = 20*3^(n-2) + a(n-2), n > 1.
a(n) = A096019(n) - 2*(-1)^n. (End)
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MATHEMATICA
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LinearRecurrence[{2, 3}, {1, 9}, 30] (* Harvey P. Dale, Jul 31 2023 *)
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PROG
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(PARI) Vec((1 + 7*x)/((1 + x)*(1 - 3*x)) + O(x^40)) \\ Andrew Howroyd, Feb 02 2020
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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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