OFFSET
2,1
LINKS
Robert Israel, Table of n, a(n) for n = 2..700
Eric Weisstein's World of Mathematics, Lattice Path.
FORMULA
a(n) = s(3, n) - s(3, floor(n/2)) * s(3, ceiling(n/2)), where s(3,n) = A006480(n).
G.f.: 4F3(1/6,1/3,2/3,5/6; 1/2,1/2,1; 729*x^2) - 4F3(1/3,1/3,2/3,2/3; 1,1,1; 729*x^2) + 6*x*( 4F3(2/3,5/6,7/6,4/3; 1,3/2,3/2; 729*x^2) - 4F3(1/3,2/3,4/3,5/3; 1,2,2; 729*x^2)). - Benedict W. J. Irwin, Oct 20 2016
MAPLE
g:= hypergeom([1/6, 1/3, 2/3, 5/6], [ 1/2, 1/2, 1], 729*x^2) - hypergeom([1/3, 1/3, 2/3, 2/3], [ 1, 1, 1], 729*x^2) + 6*x*( hypergeom([2/3, 5/6, 7/6, 4/3], [ 1, 3/2, 3/2], 729*x^2) - hypergeom([1/3, 2/3, 4/3, 5/3], [ 1, 2, 2], 729*x^2)):
S:= series(g, x, 101):
seq(coeff(S, x, j), j=2..100); # Robert Israel, Oct 20 2016
MATHEMATICA
Table[Factorial[3n]/Factorial[n]^3 - Factorial[3Floor[n/2]]Factorial[3 Ceiling[n/2]]/Factorial[Floor[n/2]]^3/Factorial[Ceiling[n/2]]^3, {n, 2, 20}]
Rest[Rest[CoefficientList[Series[HypergeometricPFQ[{1/6, 1/3, 2/3, 5/6}, {1/2, 1/2, 1}, 729 x^2] - HypergeometricPFQ[{1/3, 1/3, 2/3, 2/3}, {1, 1, 1}, 729 x^2] + 6 x (HypergeometricPFQ[{2/3, 5/6, 7/6, 4/3}, {1, 3/2, 3/2}, 729 x^2] - HypergeometricPFQ[{1/3, 2/3, 4/3, 5/3}, {1, 2, 2}, 729 x^2]), {x, 0, 20}], x]]] (* Benedict W. J. Irwin, Oct 20 2016 *)
PROG
(PARI) A006480(n)=(3*n)!/n!^3
CROSSREFS
KEYWORD
easy,nice,nonn
AUTHOR
T. D. Noe, Jun 06 2002
STATUS
approved