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A103446
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Unlabeled analog of A025168.
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9
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0, 1, 3, 8, 21, 54, 137, 344, 856, 2113, 5179, 12614, 30548, 73595, 176455, 421215, 1001388, 2371678, 5597245, 13166069, 30873728, 72185937, 168313391, 391428622, 908058205, 2101629502, 4853215947, 11183551059, 25718677187, 59030344851, 135237134812
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OFFSET
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0,3
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COMMENTS
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Or, if the initial 0 is omitted, this is the binomial transform of the partition numbers p(1), p(2), ... = 1, 2, 3, 5, 7, 11, 15, 22, 30, ... (A000041 without the initial 1).
The most precise definition of this sequence is the Maple combstruct command given below. See the first Wieder link for further details.
Sequence appears to have a rational o.g.f. - Ralf Stephan, May 18 2007
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LINKS
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FORMULA
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O.g.f.: exp( Sum_{n>=1} sigma(n)*x^n/(1-x)^n/n ) - 1. - Paul D. Hanna, Apr 21 2010
O.g.f.: exp( Sum_{n>=1} x^n/n * Sum_{k=1..n} binomial(n,k)*sigma(k) ) - 1. - Paul D. Hanna, Feb 04 2012
O.g.f. P(x/(1-x)), where P(x) is the o.g.f. for number of partitions (A000041) a(n)=sum_{k=1,n} ( binomial(n-1,k-1)*A000041(k)). - Vladimir Kruchinin, Aug 10 2010
a(n) ~ exp(Pi*sqrt(n/3) + Pi^2/24) * 2^(n-2) / (n*sqrt(3)). - Vaclav Kotesovec, Jun 25 2015
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EXAMPLE
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Let {} denote a set, [] a list and Z an unlabeled element.
a(3) = 8 because we have {[[Z]],[[Z]],[[Z]]}, {[[Z],[Z]],[[Z]]}, {[[Z],[Z],[Z]]}, {[[Z],[Z,Z]]}, {[[Z,Z],[Z]]}, {[[Z,Z]],[[Z]]}, {[[Z]],[[Z,Z]]}, {[[Z,Z,Z]]}.
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MAPLE
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with(combstruct); SubSetSeqU := [T, {T=Subst(U, S), S=Set(U, card>=1), U=Sequence(Z, card>=1)}, unlabeled]; [seq(count(SubSetSeqU, size=n), n=0..30)];
allstructs(SubSetSeq, size=3); # to get the structures for n=3 - this output is shown in the example lines.
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MATHEMATICA
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Flatten[{0, Table[Sum[Binomial[n-1, k]*PartitionsP[k+1], {k, 0, n-1}], {n, 1, 30}]}] (* Vaclav Kotesovec, Jun 25 2015 *)
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PROG
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(PARI) {a(n)=if(n<1, 0, polcoeff(exp(sum(m=1, n, sigma(m)*x^m/(1-x+x*O(x^n))^m/m)), n))} \\ Paul D. Hanna, Apr 21 2010
(PARI) {a(n)=if(n<1, 0, polcoeff(exp(sum(m=1, n, x^m/m*sum(k=1, m, binomial(m, k)*sigma(k)))+x*O(x^n)), n))} \\ Paul D. Hanna, Feb 04 2012
(PARI) Vec(1/eta('x/(1-'x)+O('x^66))) \\ Joerg Arndt, Jul 30 2011
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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I can confirm that the terms shown are the binomial transform of the partition sequence 1, 2, 3, 5, 7, 11, 15, 22, 30, 42, 56, 77, 101, ... (A000041 without the a(0) term). - N. J. A. Sloane, May 18 2007
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STATUS
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approved
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