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A024486 a(n) = (1/(2n+1))*Multinomial(3n; n,n,n). 1
1, 2, 18, 240, 3850, 68796, 1319472, 26604864, 556794810, 11993338500, 264333180540, 5935956324480, 135389270500848, 3128818447143200, 73123195376232000, 1725644509205633280, 41071074678970154010, 984853105623747983700, 23773506148264947574500 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..18.

FORMULA

From Vaclav Kotesovec, Feb 19 2015: (Start)

a(n) = (3*n)! / ((2*n+1) * (n!)^3).

D-finite with recurrence: n^2*(2*n+1)*a(n) = 3*(2*n-1)*(3*n-2)*(3*n-1)*a(n-1).

a(n) ~ 3^(3*n+1/2) / (4*Pi*n^2).

(End)

a(n) = A006480(n)/(2n+1). - R. J. Mathar, Apr 27 2020

MATHEMATICA

Table[Multinomial[n, n, n]/(2*n+1), {n, 0, 20}] (* Vaclav Kotesovec, Feb 19 2015 *)

CROSSREFS

Sequence in context: A245112 A260332 A254999 * A052635 A259270 A168562

Adjacent sequences:  A024483 A024484 A024485 * A024487 A024488 A024489

KEYWORD

nonn

AUTHOR

Clark Kimberling

EXTENSIONS

More terms from Vaclav Kotesovec, Feb 19 2015

STATUS

approved

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Last modified April 11 00:03 EDT 2021. Contains 342877 sequences. (Running on oeis4.)