A052666 - OEIS

%I #13 Apr 18 2017 07:03:59

%S 1,1,8,42,456,4800,69840,1093680,20482560,420577920,9736070400,

%T 245887488000,6806133734400,203555082931200,6565920180019200,

%U 226728504946944000,8355118608764928000,327047476385710080000

%N E.g.f. 1/(1-x-3x^2).

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=613">Encyclopedia of Combinatorial Structures 613</a>

%F E.g.f.: -1/(-1+x+3*x^2)

%F Recurrence: {a(1)=1, a(0)=1, (-3*n^2-9*n-6)*a(n)+(-2-n)*a(n+1)+a(n+2)=0}

%F Sum(1/13*(1+6*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+3*_Z^2))*n!

%F a(n) = n!*A006130(n). - _R. J. Mathar_, Nov 27 2011

%p spec := [S,{S=Sequence(Union(Z,Prod(Z,Union(Z,Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);

%K easy,nonn

%O 0,3

%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000