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5, 15, 50, 150, 455, 1365, 4100, 12300, 36905, 110715, 332150, 996450, 2989355, 8968065, 26904200, 80712600, 242137805, 726413415, 2179240250, 6537720750, 19613162255, 58839486765, 176518460300, 529555380900, 1588666142705, 4765998428115, 14297995284350
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OFFSET
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2,1
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COMMENTS
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This sequence is associated with A340131, whose terms are sorted by the length of their ternary code. Elements with the same length of ternary code form a range that has a maximum. The maximal term of the n-range (a set of elements with ternary code length n in A340131) is a(n). Example: numbers 29, 33, 44, 45 and 50 have a ternary length of 4 (see A340131), respectively a(4) = 50.
Ternary code for a(n) is 12..12 for even n and 12..120 for odd n.
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LINKS
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FORMULA
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a(n) = 5*3^(n-2*k)*(9^k-1)/8 where k = floor(n/2).
a(n+1) = 3*a(n) for even n >= 2; a(n+1) = 3*a(n)+5 for odd n >= 3.
a(n) = (5/8)*(3^n - (-1)^(n-1) - 2).
a(n) = 2*a(n-1) + 3*a(n-2) + 5 for n > 3.
G.f.: 5*x^2/(1 - 3*x - x^2 + 3*x^3).
a(n) = 3*a(n-1) + a(n-2) - 3*a(n-3) for n > 4. (End)
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EXAMPLE
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PROG
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(PARI) Vec(5/(1 - 3*x - x^2 + 3*x^3) + O(x^30)) \\ Andrew Howroyd, Jan 08 2021
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CROSSREFS
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KEYWORD
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nonn,easy,base
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AUTHOR
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STATUS
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approved
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