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%I #16 Jun 03 2022 18:56:01
%S 0,1,0,6,24,240,2160,25200,322560,4717440,76204800,1357171200,
%T 26345088000,554204851200,12553673932800,304688127744000,
%U 7887891787776000,216969331138560000,6319142847553536000
%N E.g.f. x(1-x)/(1-x-x^2).
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=527">Encyclopedia of Combinatorial Structures 527</a>
%F E.g.f.: x*(-1+x)/(-1+x+x^2)
%F Recurrence: {a(1)=1, a(0)=0, a(2)=0, (-2-n^2-3*n)*a(n)+(-2-n)*a(n+1)+a(n+2)=0}
%F Sum(-1/5*(-3+4*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2))*n!
%F a(n)=n!*A212804(n-1). - _R. J. Mathar_, Jun 03 2022
%p spec := [S,{S=Prod(Z,Sequence(Prod(Z,Z,Sequence(Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t With[{nn=20},CoefficientList[Series[x (1-x)/(1-x-x^2),{x,0,nn}], x]Range[0,nn]!] (* _Harvey P. Dale_, Jul 24 2012 *)
%K easy,nonn
%O 0,4
%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000