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A216255
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Triangle read by rows: T(n,k) is the number of labeled rooted trees of height at most 2 that have exactly k nodes at a distance 2 from the root; n>=1, 0<=k<=n-1.
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1
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1, 2, 0, 3, 6, 0, 4, 24, 12, 0, 5, 60, 120, 20, 0, 6, 120, 540, 480, 30, 0, 7, 210, 1680, 3780, 1680, 42, 0, 8, 336, 4200, 17920, 22680, 5376, 56, 0, 9, 504, 9072, 63000, 161280, 122472, 16128, 72, 0, 10, 720, 17640, 181440, 787500, 1290240, 612360, 46080, 90, 0
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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E.g.f.: x*exp(x*exp(y*x)).
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EXAMPLE
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1;
2, 0;
3, 6, 0;
4, 24, 12, 0;
5, 60, 120, 20, 0;
6, 120, 540, 480, 30, 0;
7, 210, 1680, 3780, 1680, 42, 0;
8, 336, 4200, 17920, 22680, 5376, 56, 0;
9, 504, 9072, 63000, 161280, 122472, 16128, 72, 0;
10, 720, 17640, 181440, 787500, 1290240, 612360, 46080, 90, 0;
T(4,1) = 24 because there is only one unlabeled tree on 4 nodes with exactly 1 node at distance two from the root. It has 24 labelings.
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MAPLE
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T:= (n, k)-> n*binomial(n-1, k)*(n-k-1)^k:
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MATHEMATICA
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nn=10; a=NestList[x Exp[#]&, y x, nn]; f[list_]:=Select[list, #>0&]; Map[f, Range[0, nn]!CoefficientList[Series[a[[3]], {x, 0, nn}], {x, y}]]//Grid
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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