%I #9 Apr 18 2017 07:04:02

%S 0,0,0,0,1,4,14,52,201,792,3168,12844,52676,218148,910996,3832072,

%T 16222352,69061200,295477550,1269863304,5479456290

%N A simple context-free grammar.

%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=665">Encyclopedia of Combinatorial Structures 665</a>

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

%F Recurrence: {a(1)=0, a(2)=0, a(4)=1, a(3)=0, a(5)=4, a(6)=14, (2304-4608*n-36864*n^2+73728*n^3)*a(n)+(46368+121536*n+59904*n^2-9216*n^3)*a(n+1)+(-79800-56512*n+59520*n^2+34048*n^3)*a(n+2)+(200516*n+185964+68544*n^2+7456*n^3)*a(n+3)+(-15024*n^2-22732*n+6864-2228*n^3)*a(n+4)+(-217*n^3-2604*n^2-10199*n-13020)*a(n+5)}

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

%K easy,nonn

%O 0,6

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