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A052689
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E.g.f. (1+x-x^2)/((1-x)(1-x^2)).
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0
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1, 2, 4, 18, 72, 480, 2880, 25200, 201600, 2177280, 21772800, 279417600, 3353011200, 49816166400, 697426329600, 11769069312000, 188305108992000, 3556874280960000, 64023737057280000, 1338096104497152000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 637
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FORMULA
| E.g.f.: -(-x+x^2-1)/(-1+x)/(-1+x^2)
Recurrence: {a(0)=1, a(2)=4, a(1)=2, (-14*n-n^3-7*n^2-8)*a(n)+(-2-n)*a(n+1)+(n+3)*a(n+2)=0}
(1/4*(-1)^(1-n)+1/2*n+5/4)*n!.
a(n) = n!*A004526(n+3). - R. J. Mathar, Nov 27 2011
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MAPLE
| spec := [S, {S=Prod(Union(Z, Sequence(Z)), Sequence(Prod(Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A179040 A009679 A007727 * A139104 A014448 A075836
Adjacent sequences: A052686 A052687 A052688 * A052690 A052691 A052692
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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