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A071208
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Triangular array T(n,k) read by rows, giving number of labeled free trees such that the root is smaller than all its children, with respect to the number n of vertices and to the number k of decreasing edges.
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1
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1, 2, 2, 6, 15, 6, 24, 104, 104, 24, 120, 770, 1345, 770, 120, 720, 6264, 16344, 16344, 6264, 720, 5040, 56196, 200452, 300167, 200452, 56196, 5040, 40320, 554112, 2552192, 5241984, 5241984, 2552192, 554112, 40320, 362880, 5973264, 34138908, 90857052, 124756281, 90857052, 34138908, 5973264, 362880
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OFFSET
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1,2
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LINKS
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FORMULA
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T(n,k) = Sum_{m=k+1..n} (-1)^(k+1)*binomial(m,k+1)*Stirling1(n+1,n+1-m)*n^(n-m) with 0 <= k < n.
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EXAMPLE
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Triangle begins:
1
2 2
6 15 6
24 104 104 24
120 770 1345 770 120
720 6264 16344 16344 6264 720
...
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MAPLE
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T:= (n, k)-> add((-1)^(k+1)*binomial(m, k+1)*
Stirling1(n+1, n+1-m)*n^(n-m), m=k+1..n):
seq(seq(T(n, k), k=0..n-1), n=1..9);
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MATHEMATICA
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A071208[n_, k_] := Sum[(-1)^(k+1)*Binomial[m, k+1]*StirlingS1[n+1, n+1-m]*n^(n-m), {m, k+1, n}];
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CROSSREFS
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KEYWORD
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AUTHOR
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Cedric Chauve (chauve(AT)lacim.uqam.ca), May 16 2002
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EXTENSIONS
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STATUS
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approved
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