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A055533 Number of labeled order relations on n nodes in which longest chain has n-1 nodes. 6
1, 12, 108, 960, 9000, 90720, 987840, 11612160, 146966400, 1995840000, 28979596800, 448345497600, 7366565606400, 128152088064000, 2353813862400000, 45527990796288000, 925143000477696000 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

COMMENTS

Also number of labeled acyclic digraphs with n nodes and binomial(n,2)-1 arcs, cf. A081064. - Vladeta Jovovic, Jul 14 2008

REFERENCES

L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 60.

LINKS

Table of n, a(n) for n=2..18.

FORMULA

E.g.f.: (x^2/2)(1+x)/(1-x)^3. a(n)=n!(n-1)^2/2, if n>1. - Vladeta Jovovic, Dec 01 2002

PROG

(PARI) a(n)=if(n<2, 0, n!/2*(n-1)^2) /* Michael Somos, Mar 06 2004 */

CROSSREFS

A column or diagonal of triangle in A008285.

Sequence in context: A037972 A111990 A053469 * A037602 A037707 A140317

Adjacent sequences:  A055530 A055531 A055532 * A055534 A055535 A055536

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Jul 10 2000

EXTENSIONS

More terms from Vladeta Jovovic, Dec 01 2002

STATUS

approved

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Last modified October 18 11:56 EDT 2018. Contains 316321 sequences. (Running on oeis4.)