%I #11 Apr 18 2017 07:04:03

%S 0,0,2,12,120,1920,40320,1028160,30965760,1077753600,42551308800,

%T 1878484608000,91686654259200,4902558383923200,284994551761920000,

%U 17895345882365952000,1207064057173438464000,87041570211234422784000

%N A simple context-free grammar in a labeled universe.

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=694">Encyclopedia of Combinatorial Structures 694</a>

%F E.g.f.: RootOf(-_Z+_Z^4+_Z^2+x)^2

%F Recurrence: a(1)=0; a(2)=2; a(3)=12; (9216*n^3-2496*n-576+1152*n^2+6144*n^4)*a(n) +(-9096-30320*n-28032*n^2-7936*n^3)*a(n+1) +(15488*n+6272*n^2+10404)*a(n+2) +(-3784*n-7600)*a(n+3) +589*a(n+4)=0; a(4)=120.

%F a(n) = n!*A052708(n). - _R. J. Mathar_, Oct 16 2013

%p spec := [S,{B=Union(S,Z,C),S=Prod(B,B),C=Prod(S,S)},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