OFFSET
0,2
FORMULA
a(n) = 9^n * Sum_{k=0..n} binomial(n,k) * binomial(n/3+k/3+1/3,n)/(n+k+1).
PROG
(PARI) a(n) = 9^n*sum(k=0, n, binomial(n, k)*binomial(n/3+k/3+1/3, n)/(n+k+1));
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Apr 15 2024
STATUS
approved