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A052703
A simple context-free grammar.
1
0, 0, 0, 1, 3, 12, 49, 210, 927, 4191, 19305, 90285, 427570, 2046324, 9881862, 48090824, 235619133, 1161257580, 5753365015, 28638093270, 143148398085
OFFSET
0,5
FORMULA
G.f.: RootOf(-_Z+_Z^2+_Z^3+x)-RootOf(-_Z+_Z^2+_Z^3+x)^2-x
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)}
MAPLE
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);
CROSSREFS
Sequence in context: A012864 A045890 A049673 * A151170 A151171 A151172
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved