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A111246
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Triangle read by rows: a(n,k) = number of partitions of an n-set into exactly k nonempty subsets, each of size <= 3.
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7
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1, 1, 1, 1, 3, 1, 0, 7, 6, 1, 0, 10, 25, 10, 1, 0, 10, 75, 65, 15, 1, 0, 0, 175, 315, 140, 21, 1, 0, 0, 280, 1225, 980, 266, 28, 1, 0, 0, 280, 3780, 5565, 2520, 462, 36, 1, 0, 0, 0, 9100, 26145, 19425, 5670, 750, 45, 1, 0, 0, 0, 15400, 102025, 125895, 56595, 11550, 1155
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OFFSET
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1,5
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COMMENTS
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a(n,k) = 0 if k > n; a(n,k) = 0 if n > 0 and k < 0; a(n,k) can be extended to negative n and k, just as the Stirling numbers or Pascal's triangle can be extended. The present triangle is called the tri-restricted Stirling numbers of the second kind.
Also the Bell transform of the sequence "a(n) = 1 if n<3 else 0". For the definition of the Bell transform see A264428. - Peter Luschny, Jan 27 2016
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REFERENCES
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J. Y. Choi and J. D. H. Smith, On the combinatorics of multi-restricted numbers, Ars. Com., 75(2005), pp. 44-63.
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LINKS
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FORMULA
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a(n, k) = a(n-1, k-1) + k*a(n-1, k) - binomial(n-1, 3)*a(n-4, k-1).
G.f. = Sum_{k_1+k_2+k_3=k, k_1+ 2k_2+3k_3=n} frac{n!}{(1!)^{k_1}(2!)^{k_2}(3!)^{k_3}k_1!k_2!k_3!}.
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EXAMPLE
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a(1,1)=1;
a(2,1)=1; a(2,2)=1;
a(3,1)=1; a(3,2)=3; a(3,3)=1;
a(4,1)=0; a(4,2)=7; a(4,3)=6; a(4,4)=1;
a(5,1)=0; a(5,2)=10; a(5,3)=25; a(5,4)=10; a(5,5)=1;
a(6,1)=0; a(6,2)=10; a(6,3)=75; a(6,4)=65; a(6,5)=15; a(6,6)=1; ...
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MAPLE
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# The function BellMatrix is defined in A264428.
# Adds (1, 0, 0, 0, ...) as column 0.
BellMatrix(n -> `if`(n<3, 1, 0), 10); # Peter Luschny, Jan 27 2016
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MATHEMATICA
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BellMatrix[f_Function, len_] := With[{t = Array[f, len, 0]}, Table[BellY[n, k, t], {n, 0, len - 1}, {k, 0, len - 1}]];
rows = 12;
M = BellMatrix[If[# < 3, 1, 0]&, rows];
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PROG
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(PARI) row(n) = {x='x+O('x^(n+1)); polcoeff(serlaplace(exp(y*(x+x^2/2+x^3/6))), n, 'x); }
tabl(nn) = for(n=1, nn, print(Vecrev(row(n)/y))) \\ Jinyuan Wang, Dec 21 2019
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CROSSREFS
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KEYWORD
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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