OFFSET
0,2
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..431
Robert W. Donley Jr, Directed path enumeration for semi-magic squares of size three, arXiv:2107.09463 [math.CO], 2021.
FORMULA
a(n) ~ 216^n / (Pi*n)^2. - Vaclav Kotesovec, Jun 21 2021
MATHEMATICA
Array[Sum[(3 #)!/(k!*(# - k)!)^3, {k, 0, #}] &, 15, 0] (* Michael De Vlieger, Dec 02 2021 *)
PROG
(PARI) {a(n) = sum(k=0, n, (3*n)!/(k!*(n-k)!)^3)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Mar 02 2019
STATUS
approved