A024488 a(n) = (1/(3n-1))*M(3n; n,n,n), where M(...) is a multinomial coefficient.

3, 18, 210, 3150, 54054, 1009008, 19953648, 411543990, 8764362750, 191413682460, 4266468608220, 96706621786320, 2223107844022800, 51721284534408000, 1215794995122150720, 28837137540553512390

Offset

1,1

Links

Table of n, a(n) for n=1..16.

Formula

a(n) ~ 3^(3*n-1/2) / (2*Pi*n^2). - Vaclav Kotesovec, Feb 19 2015

a(n) = A006480(n)/(3*n-1) = 3*A024489(n). - R. J. Mathar, Apr 27 2020

D-finite with recurrence: n^2*a(n) -3*(3*n-2)*(3*n-4)*a(n-1)=0. - R. J. Mathar, Apr 27 2020

Mathematica

Table[Multinomial[n, n, n]/(3*n-1), {n, 1, 20}] (* Vaclav Kotesovec, Feb 19 2015 *)

Crossrefs

Sequence in context: A277355 A135077 A157538 * A217902 A356614 A183241

Adjacent sequences: A024485 A024486 A024487 * A024489 A024490 A024491

Keyword

nonn,easy

Author

Clark Kimberling

Extensions

More terms from James A. Sellers, Aug 08 2000

Status

approved