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A120348 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). 1
1, 8, 156, 4896, 212520, 11793600, 797448960, 63606090240, 5846743244160, 608588457523200, 70758332701056000, 9088747467351552000, 1278179579224720972800, 195333707771834926694400 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

E. M. Palmer and R. C. Read, On the number of plane 2-trees, J. London Math. Soc. (2), 6, 1973, 583-592.

LINKS

Table of n, a(n) for n=1..14.

FORMULA

a(n) = (5n-2)!/(4n-1)!.

E.g.f. T = T(x) satisfies T(1-T)^4 = x.

MAPLE

seq((5*n-2)!/(4*n-1)!, n=1..16);

CROSSREFS

Sequence in context: A288682 A268543 A113668 * A251586 A221098 A171211

Adjacent sequences:  A120345 A120346 A120347 * A120349 A120350 A120351

KEYWORD

nonn

AUTHOR

Emeric Deutsch, Jun 24 2006

STATUS

approved

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Last modified August 19 05:01 EDT 2019. Contains 326109 sequences. (Running on oeis4.)