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A129532
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3n(n-1)4^(n-2).
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4
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0, 0, 6, 72, 576, 3840, 23040, 129024, 688128, 3538944, 17694720, 86507520, 415236096, 1962934272, 9160359936, 42278584320, 193273528320, 876173328384, 3942779977728, 17626545782784, 78340203479040, 346346162749440
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OFFSET
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0,3
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COMMENTS
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Number of inversions in all 4-ary words of length n on {0,1,2,3}. Example: a(2)=6 because each of the words 10,20,30,21,31,32 has one inversion and the words 00,01,02,03,11,12,13,22,23,33 have no inversions. a(n)=Sum(k*A129531(n,k),k>=0). a(n)=6*A038845(n-2).
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LINKS
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FORMULA
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G.f.=6x^2/(1-4x)^3.
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MAPLE
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seq(3*n*(n-1)*4^(n-2), n=0..25);
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MATHEMATICA
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Table[3n(n-1)4^(n-2), {n, 0, 30}] (* or *) LinearRecurrence[{12, -48, 64}, {0, 0, 6}, 30] (* Harvey P. Dale, May 25 2018 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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