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%I #16 Apr 18 2017 07:03:55
%S 0,0,2,0,24,120,1440,15120,201600,2903040,47174400,838252800,
%T 16286054400,342486144000,7758867916800,188305108992000,
%U 4875010043904000,134094160392192000,3905447960494080000
%N E.g.f. x^2*(1-x)/(1-x-x^2).
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=547">Encyclopedia of Combinatorial Structures 547</a>
%F E.g.f.: x^2*(-1+x)/(-1+x+x^2)
%F Recurrence: {a(1)=0, a(0)=0, a(3)=0, a(2)=2, (-2-n^2-3*n)*a(n) +(-2-n)*a(n+1) +a(n+2)=0}
%F Sum(1/5*(-4+7*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2))*n!
%p spec := [S,{S=Prod(Z,Z, Sequence(Prod(Z,Z, Sequence(Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t With[{nn=30},CoefficientList[Series[(x^2*(1-x))/(1-x-x^2),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Dec 21 2011 *)
%K easy,nonn
%O 0,3
%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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