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A219614
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Number of ways to put n labeled objects into n labeled boxes so that no two nonempty boxes are adjacent.
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1
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1, 1, 2, 9, 46, 335, 2786, 28357, 325382, 4280859, 62437882, 1010306825, 17852477006, 343275422503, 7120802805650, 158697470231757, 3778977532041430, 95794295907958547, 2574920565897373610, 73164585387874543057, 2191028450841437523230, 68974613397532849153311
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = Sum_{k>=0} binomial(n-k+1,k)*Stirling2(n,k)*k!.
Limit n->infinity (a(n)/n!)^(1/n) = (3*r^2-3*r+1)/(1-2*r) = 1.53445630931668421506236..., where r = 0.410751485627... is the root of the equation (1-2*r)^2 + r*(1-3*r+3*r^2)*LambertW(-exp(-1/r)/r) = 0. - Vaclav Kotesovec, Dec 08 2012
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EXAMPLE
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a(3) = 9 because we have: (1,1,3), (1,3,1), (1,3,3), (3,1,1), (3,1,3), (3,3,1), (1,1,1), (2,2,2), (3,3,3).
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MAPLE
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with (combinat):
a:= n-> add(stirling2(n, k)*k! *binomial(n-k+1, k), k=0..ceil(n/2)):
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MATHEMATICA
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Table[Sum[Binomial[n-k+1, k]StirlingS2[n, k]k!, {k, 0, n}], {n, 0, 20}]
(* Program for numerical value of the limit (a(n)/n!)^(1/n) *) (3*r^2-3*r+1)/(1-2*r)/.FindRoot[(1-2*r)^2+r*(1-3*r+3*r^2)*LambertW[-E^(-1/r)/r]==0, {r, 1/2}, WorkingPrecision->100] (* Vaclav Kotesovec, Dec 08 2012 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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