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E.g.f. 2*x^2*(1+x-x^2)/(1-x).
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%I #13 May 14 2017 08:12:30

%S 0,0,4,24,48,240,1440,10080,80640,725760,7257600,79833600,958003200,

%T 12454041600,174356582400,2615348736000,41845579776000,

%U 711374856192000,12804747411456000,243290200817664000

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

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=591">Encyclopedia of Combinatorial Structures 591</a>

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

%F Recurrence: {a(1)=0, a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(2)=4, a(3)=24, a(4)=48}

%F 2*n!, n>3.

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

%t With[{nn=20},CoefficientList[Series[2x^2 (1+x-x^2)/(1-x),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, May 14 2017 *)

%K easy,nonn

%O 0,3

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