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A171761
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Number of ternary squareful words of length n.
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0
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0, 0, 3, 15, 63, 213, 687, 2127, 6483, 19575, 58905, 176943, 531177, 1593981, 4782513, 14348289, 43045923, 129139119, 387419097, 1162259637, 3486782013, 10460350023, 31381055463, 94143173409
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OFFSET
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0,3
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LINKS
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Table of n, a(n) for n=0..23.
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FORMULA
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a(n) = A000244(n) - A006156(n).
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MATHEMATICA
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nmax = 23; 3^Range[0, nmax] - Length /@ NestList[ DeleteCases[ Flatten[ Outer[ Append, #, {1, 2, 3}, 1], 1], {___, x__, x__, ___}] &, {{}}, nmax] (* Jean-François Alcover, Jun 12 2017, using Vladimir Reshetnikov's code for A006156 *)
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CROSSREFS
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Cf. A000244, A006156.
Sequence in context: A062211 A260326 A204086 * A229277 A218313 A218190
Adjacent sequences: A171758 A171759 A171760 * A171762 A171763 A171764
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KEYWORD
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nonn
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AUTHOR
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Mats Granvik, Dec 18 2009
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STATUS
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approved
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