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A080599
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Expansion of e.g.f.: 2/(2-2*x-x^2).
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20
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1, 1, 3, 12, 66, 450, 3690, 35280, 385560, 4740120, 64751400, 972972000, 15949256400, 283232149200, 5416632421200, 110988861984000, 2425817682288000, 56333385828720000, 1385151050307024000, 35950878932544576000, 982196278209226080000, 28175806418228108640000
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OFFSET
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0,3
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COMMENTS
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Number of ordered partitions of {1,..,n} with at most 2 elements per block. - Bob Proctor, Apr 18 2005
In other words, number of preferential arrangements of n things (see A000670) in which each clump has size 1 or 2. - N. J. A. Sloane, Apr 13 2014
Recurrences (of the hypergeometric type of the Jovovic formula) mean: multiplying the sequence vector from the left with the associated matrix of the recurrence coefficients (here: an infinite lower triangular matrix with the natural numbers in the main diagonal and the triangular series in the subdiagonal) recovers the sequence up to an index shift. In that sense, this sequence here and many other sequences of the OEIS are eigensequences. - Gary W. Adamson, Feb 14 2011
Number of intervals in the weak (Bruhat) order of S_n that are Boolean algebras. - Richard Stanley, May 09 2011
Also the number of (1,1,1)-avoiding or cubefree sequences of length n covering an initial interval of positive integers. For example, the a(0) = 1 through a(3) = 12 sequences are:
() (1) (11) (112)
(12) (121)
(21) (122)
(123)
(132)
(211)
(212)
(213)
(221)
(231)
(312)
(321)
(End)
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LINKS
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FORMULA
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a(n) = n!*((1+sqrt(3))^(n+1) - (1-sqrt(3))^(n+1))/(2^(n+1)*sqrt(3)). - Vladimir Reshetnikov, Oct 31 2015
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EXAMPLE
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The a(0) = 1 through a(3) = 12 ordered set partitions with block sizes <= 2 are:
{} {{1}} {{1,2}} {{1},{2,3}}
{{1},{2}} {{1,2},{3}}
{{2},{1}} {{1,3},{2}}
{{2},{1,3}}
{{2,3},{1}}
{{3},{1,2}}
{{1},{2},{3}}
{{1},{3},{2}}
{{2},{1},{3}}
{{2},{3},{1}}
{{3},{1},{2}}
{{3},{2},{1}}
(End)
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MAPLE
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a:= n-> n! *(Matrix([[1, 1], [1/2, 0]])^n)[1, 1]:
a:= gfun:-rectoproc({a(n) = n*a(n-1)+(n*(n-1)/2)*a(n-2), a(0)=1, a(1)=1}, a(n), remember):
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MATHEMATICA
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Table[n!*SeriesCoefficient[-2/(-2+2*x+x^2), {x, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Oct 13 2012 *)
Round@Table[n! ((1+Sqrt[3])^(n+1) - (1-Sqrt[3])^(n+1))/(2^(n+1) Sqrt[3]), {n, 0, 20}] (* Vladimir Reshetnikov, Oct 31 2015 *)
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PROG
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(PARI) Vec(serlaplace((2/(2-2*x-x^2) + O(x^30)))) \\ Michel Marcus, Nov 02 2015
(Magma) [n le 2 select 1 else (n-1)*Self(n-1) + Binomial(n-1, 2)*Self(n-2): n in [1..31]]; // G. C. Greubel, Jan 31 2023
(SageMath)
A002605=BinaryRecurrenceSequence(2, 2, 0, 1)
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CROSSREFS
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(1,1)-avoiding patterns are A000142.
(1,1,1)-avoiding compositions are A232432.
(1,1,1)-matching patterns are A335508.
(1,1,1)-avoiding permutations of prime indices are A335511.
(1,1,1)-avoiding compositions are ranked by A335513.
(1,1,1,1)-avoiding patterns are A189886.
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KEYWORD
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nonn,eigen
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AUTHOR
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Detlef Pauly (dettodet(AT)yahoo.de), Feb 24 2003
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STATUS
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approved
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