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 A052621 E.g.f. (2+x+x^2+x^3)/(1-x^4). 0
 2, 1, 2, 6, 48, 120, 720, 5040, 80640, 362880, 3628800, 39916800, 958003200, 6227020800, 87178291200, 1307674368000, 41845579776000, 355687428096000, 6402373705728000, 121645100408832000, 4865804016353280000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 567 FORMULA E.g.f.: -(x^3+x^2+x+2)/(-1+x^4) Recurrence: {a(1)=1, a(3)=6, a(2)=2, a(0)=2, (-n^4-35*n^2-50*n-24-10*n^3)*a(n)+a(n+4)=0} Sum(1/4*(_alpha^3+_alpha^2+2*_alpha+1)*_alpha^(-1-n), _alpha=RootOf(-1+_Z^4))*n! 2n! if n is 0 mod 4, n! otherwise. MAPLE spec := [S, {S=Union(Sequence(Z), Sequence(Prod(Z, Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); CROSSREFS Sequence in context: A038078 A000139 A114572 * A212671 A131057 A051852 Adjacent sequences:  A052618 A052619 A052620 * A052622 A052623 A052624 KEYWORD easy,nonn AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 STATUS approved

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