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A120348
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Number of labeled simply-rooted 2-trees with n labeled vertices (i.e. n+2 vertices altogether; a simply-rooted 2-tree is an externally rooted 2-tree whose root edge belongs to exactly one triangle).
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0
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1, 8, 156, 4896, 212520, 11793600, 797448960, 63606090240, 5846743244160, 608588457523200, 70758332701056000, 9088747467351552000, 1278179579224720972800, 195333707771834926694400
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| E. M. Palmer and R. C. Read, On the number of plane 2-trees, J. London Math. Soc. (2), 6, 1973, 583-592.
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FORMULA
| a(n)=(5n-2)!/(4n-1)! E.g.f. T=T(x) satisfies T(1-T)^4=x.
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MAPLE
| seq((5*n-2)!/(4*n-1)!, n=1..16);
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CROSSREFS
| Sequence in context: A188408 A089669 A113668 * A171211 A025605 A114223
Adjacent sequences: A120345 A120346 A120347 * A120349 A120350 A120351
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KEYWORD
| nonn
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AUTHOR
| Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 24 2006
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