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A052616
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E.g.f. (3+2x)/(1-x^2).
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0
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3, 2, 6, 12, 72, 240, 2160, 10080, 120960, 725760, 10886400, 79833600, 1437004800, 12454041600, 261534873600, 2615348736000, 62768369664000, 711374856192000, 19207121117184000, 243290200817664000
(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 561
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FORMULA
| E.g.f.: -(2*x+3)/(-1+x^2)
Recurrence: {a(1)=2, a(0)=3, (-2-n^2-3*n)*a(n)+a(n+2)=0}
Sum(1/2*(3*_alpha+2)*_alpha^(-1-n), _alpha=RootOf(-1+_Z^2))*n!
3n! if n is even, 2n! otherwise.
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MAPLE
| spec := [S, {S=Union(Sequence(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: A095011 A188621 A175182 * A091461 A078091 A073883
Adjacent sequences: A052613 A052614 A052615 * A052617 A052618 A052619
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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