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A052896
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E.g.f.: (exp(exp(x)-1)-1)^2.
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1
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0, 0, 2, 12, 64, 350, 2024, 12460, 81638, 567888, 4180848, 32470834, 265219332, 2271692124, 20350705418, 190216812260, 1850993707960, 18714559108142, 196237054861920, 2130518566431620, 23912733627261670, 277078872201375976, 3310142647325149512
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OFFSET
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0,3
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COMMENTS
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Previous name was: A simple grammar.
a(n) is the number of ways to place n labeled balls into unlabeled (but two-colored) boxes so that at least one box is red and one box is blue. - Geoffrey Critzer, Oct 16 2011
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LINKS
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FORMULA
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E.g.f.: exp(exp(x)-1)^2 - 2*exp(exp(x)-1) + 1.
For n >= 1: a(n) = Sum_{k=0...n} Stirling2(n,k)*(2^k-2) where Stirling2(n,k) is the number of set partitions of {1,2,...,n} into exactly k blocks (A008277).
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MAPLE
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spec := [S, {B=Set(Z, 1 <= card), C=Set(B, 1 <= card), S=Prod(C, C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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MATHEMATICA
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a=Exp[Exp[x]-1]; Range[0, 20]! CoefficientList[Series[(a-1)^2, {x, 0, 20}], x]
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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EXTENSIONS
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STATUS
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approved
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