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2, 4, 20, 168, 1968, 29280, 525600, 11027520, 264579840, 7142929920, 214280640000, 7071181286400, 254561568307200, 9927888709939200, 416971151460864000, 18763699200390144000, 900657519773147136000
(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 516
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FORMULA
| E.g.f.: -2*(-1+2*x)/(-1+x)/(-1+3*x)
Recurrence: {a(1)=4, a(0)=2, (3*n^2+9*n+6)*a(n)+(-4*n-8)*a(n+1)+a(n+2)=0}
(1+3^n)*n!
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MAPLE
| spec := [S, {S=Union(Sequence(Z), Sequence(Union(Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A153229 A013329 A102087 * A110371 A120388 A061348
Adjacent sequences: A052570 A052571 A052572 * A052574 A052575 A052576
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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