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Search: id:A052506
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| A052506 |
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E.g.f.: exp(x*exp(x)-x) |
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+0
1
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| 1, 0, 2, 3, 16, 65, 336, 1897, 11824, 80145, 586000, 4588001, 38239224, 337611001, 3144297352, 30779387745, 315689119456, 3383159052833, 37790736663456, 439036039824193, 5294386116882280
(list; graph; listen)
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OFFSET
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0,3
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COMMENT
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Maps with f^2=f; trees of height at most 1.
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 39
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FORMULA
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a(n) = Sum_{k=0..n} binomial(n, k)*(n-k-1)^k. - Vladeta Jovovic (vladeta(AT)eunet.rs), Apr 12 2003
a(n) = Sum_{k=0..floor(n/2)} binomial(n, k)*k!*Stirling2(n-k, k). - Vladeta Jovovic (vladeta(AT)eunet.rs), Dec 19 2004
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MAPLE
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Maps spec := [S, {S=Set(Tree), Tree=Prod(Z, Set(Z, 0 < card))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
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Cf. A000248.
Sequence in context: A012700 A012705 A103331 this_sequence A052858 A073997 A007118
Adjacent sequences: A052503 A052504 A052505 this_sequence A052507 A052508 A052509
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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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