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A052824
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A simple grammar: cycles of pairs of cycles.
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0
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0, 0, 2, 6, 34, 220, 1808, 17388, 194724, 2478096, 35418192, 561533280, 9785418432, 185921027136, 3825633439392, 84756646285920, 2011657535668128, 50924796197369088, 1369659967551038976
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 789
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FORMULA
| E.g.f.: ln(-1/(-1+ln(-1/(-1+x))^2))
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MAPLE
| spec := [S, {B=Cycle(Z), C=Prod(B, B), S=Cycle(C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A003499 A018953 A009199 * A019029 A019032 A108424
Adjacent sequences: A052821 A052822 A052823 * A052825 A052826 A052827
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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