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

%S 0,0,0,0,24,360,6480,141120,3628800,107775360,3632428800,136994457600,

%T 5716405094400,261472603392000,13009267682611200,699459327350784000,

%U 40413205580267520000,2497069798642298880000

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

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=697">Encyclopedia of Combinatorial Structures 697</a>

%F E.g.f.: (1/2-x-1/2*(1-4*x)^(1/2))*(1/2-1/2*(1-4*x)^(1/2))*x

%F Recurrence: {a(1)=0, a(2)=0, a(3)=0, a(4)=24, a(5)=360, (-22*n+2*n^2-20+4*n^3)*a(n) +(-5*n^2+6-7*n)*a(n+1) +(n+1)*a(n+2) =0.

%F a(n) = n!*A000245(n-3). - _R. J. Mathar_, Oct 18 2013

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

%K easy,nonn

%O 0,5

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