%I #9 Apr 18 2017 07:04:01
%S 0,0,0,1,3,12,49,210,927,4191,19305,90285,427570,2046324,9881862,
%T 48090824,235619133,1161257580,5753365015,28638093270,143148398085
%N A simple context-free grammar.
%H INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=656">Encyclopedia of Combinatorial Structures 656</a>
%F G.f.: RootOf(-_Z+_Z^2+_Z^3+x)-RootOf(-_Z+_Z^2+_Z^3+x)^2-x
%F Recurrence: {a(1)=0, a(2)=0, a(3)=1, a(4)=3, (30-135*n+135*n^2)*a(n)+(-130-107*n+29*n^2)*a(n+1)+(-281*n-198-91*n^2)*a(n+2)+(15*n^2+75*n+90)*a(n+3)}
%p spec := [S,{C=Prod(B,B),B=Union(S,C,Z),S=Prod(B,C)},unlabeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K easy,nonn
%O 0,5
%A encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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