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 A052668 E.g.f. 1/(1-3x-x^3). 0
 1, 3, 18, 168, 2088, 32400, 603360, 13109040, 325503360, 9092684160, 282219033600, 9635476435200, 358879494758400, 14480588157235200, 629228583138355200, 29295027261916416000, 1454816084780298240000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 616 FORMULA E.g.f.: -1/(-1+3*x+x^3) Recurrence: {a(0)=1, a(1)=3, a(2)=18, (-11*n-6-n^3-6*n^2)*a(n)+(-3*n-9)*a(n+2)+a(n+3)=0} Sum(1/15*(4+_alpha+2*_alpha^2)*_alpha^(-1-n), _alpha=RootOf(-1+3*_Z+_Z^3))*n!. a(n)=n!*A052541(n). - R. J. Mathar, Nov 27 2011 MAPLE spec := [S, {S=Sequence(Union(Z, Z, Z, Prod(Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); CROSSREFS Sequence in context: A319938 A053513 A138211 * A224788 A121423 A074932 Adjacent sequences:  A052665 A052666 A052667 * A052669 A052670 A052671 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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Last modified May 29 20:42 EDT 2020. Contains 334710 sequences. (Running on oeis4.)