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A052660
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E.g.f. (2-2x-x^2)/((1-x)(1-x-x^2)).
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0
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2, 2, 6, 24, 144, 1080, 10080, 110880, 1411200, 20321280, 326592000, 5787936000, 112086374400, 2353813862400, 53265935923200, 1291982275584000, 33434618241024000, 919452001628160000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 607
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FORMULA
| E.g.f.: -(-2+x^2+2*x)/(-1+x)/(-1+x+x^2)
Recurrence: {a(1)=2, a(2)=6, a(0)=2, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0}
(1+Sum(1/5*(1+2*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2)))*n!
a(n) = n!*A001611(n+1). - R. J. Mathar, Nov 27 2011
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MAPLE
| spec := [S, {S=Union(Sequence(Z), Sequence(Union(Z, Prod(Z, Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A188962 A076741 A093453 * A135407 A076726 A032272
Adjacent sequences: A052657 A052658 A052659 * A052661 A052662 A052663
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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