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A052703
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A simple context-free grammar.
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0
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0, 0, 0, 1, 3, 12, 49, 210, 927, 4191, 19305, 90285, 427570, 2046324, 9881862, 48090824, 235619133, 1161257580, 5753365015, 28638093270, 143148398085
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,5
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 656
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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)}
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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);
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CROSSREFS
| Sequence in context: A012864 A045890 A049673 * A151170 A151171 A151172
Adjacent sequences: A052700 A052701 A052702 * A052704 A052705 A052706
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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