%I #12 Apr 18 2017 07:04:03
%S 0,0,2,12,168,2880,64080,1723680,54633600,1992936960,82261267200,
%T 3790579161600,192895381324800,10744251136819200,650181362358528000,
%U 42476922345521664000,2979716339168464896000,223385082959833546752000
%N A simple context-free grammar in a labeled universe.
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=684">Encyclopedia of Combinatorial Structures 684</a>
%F E.g.f.: (1/2)/(x^2+1+2*x)*(1-2*x-2*x^2-(1-4*x-4*x^2)^(1/2))
%F Recurrence: {a(1)=0, a(2)=2, a(3)=12, (-4*n^3-16*n^2-20*n-8)*a(n) +(-8*n^2-26*n-20)*a(n+1) +(-2-3*n)*a(n+2) +a(n+3) =0.
%F a(n) = n!*A052705(n). - _R. J. Mathar_, Oct 18 2013
%p spec := [S,{B=Union(S,Z,C),S=Prod(B,B),C=Prod(S,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