A026032 a(n) = C(3n,n) - C(3n,n-4).

1, 3, 15, 84, 494, 2988, 18411, 114950, 724845, 4606095, 29451240, 189264672, 1221417360, 7910510312, 51388786107, 334716829674, 2185180379179, 14294808917025, 93680975707935, 614925987859260, 4042236129950970

Offset: 0

Author

Clark Kimberling

Keywords

• nonn

Crossrefs

a(n) = T(3n, n), where T is defined in A026022.

Programs

Mathematica

Table[Binomial[3n,n]-Binomial[3n,n-4],{n,0,20}] (* Harvey P. Dale, Dec 14 2012 *)

PARI

a(n) = binomial(3*n,n) - binomial(3*n,n-4); \\ Michel Marcus, May 10 2020

Formula

G.f.: (2*g-1)*(2*g^2-2*g+1)/((3*g-1)*(g-1)^4) where g*(1-g)^2 = x. - Mark van Hoeij, Nov 09 2011

Extensions

More terms from Ralf Stephan, Jan 09 2005