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A052685
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E.g.f. (1-x^2)/(1-x-2x^2+x^4).
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0
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1, 1, 4, 24, 168, 1680, 18720, 252000, 3830400, 65681280, 1251936000, 26225337600, 599710003200, 14851444608000, 396138155212800, 11320537003776000, 345079573622784000, 11176410365632512000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 633
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FORMULA
| E.g.f.: -(-1+x^2)/(1-2*x^2+x^4-x)
Recurrence: {a(1)=1, a(0)=1, a(2)=4, a(3)=24, (n^4+35*n^2+50*n+24+10*n^3)*a(n)+(-2*n^2-14*n-24)*a(n+2)+(-n-4)*a(n+3)+a(n+4)=0}
Sum(-1/283*(-112*_alpha+48*_alpha^3-9*_alpha^2-27)*_alpha^(-1-n), _alpha=RootOf(1-2*_Z^2+_Z^4-_Z))*n!
a(n) = n!*A052535(n). - R. J. Mathar, Nov 27 2011
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MAPLE
| spec := [S, {S=Sequence(Prod(Z, Union(Z, Sequence(Prod(Z, Z)))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A069722 A027079 A188913 * A032349 A103334 A156017
Adjacent sequences: A052682 A052683 A052684 * A052686 A052687 A052688
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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