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%I #13 Jan 06 2022 16:36:43
%S 0,1,8,24,192,1920,23040,322560,5160960,92897280,1857945600,
%T 40874803200,980995276800,25505877196800,714164561510400,
%U 21424936845312000,685597979049984000,23310331287699456000
%N E.g.f. x*(1+2x-4x^2)/(1-2x).
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=603">Encyclopedia of Combinatorial Structures 603</a>
%F E.g.f.: x*(-2*x+4*x^2-1)/(-1+2*x)
%F Recurrence: {a(1)=1, a(0)=0, (-2*n-2)*a(n)+a(n+1)=0, a(2)=8, a(3)=24}
%F 2^(n-1)*n!, n>2.
%p spec := [S,{S=Prod(Z,Union(Z,Z,Sequence(Union(Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t With[{nn=20},CoefficientList[Series[x (1+2x-4x^2)/(1-2x),{x,0,nn}],x] Range[ 0,nn]!] (* _Harvey P. Dale_, Jan 06 2022 *)
%K easy,nonn
%O 0,3
%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000