OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..n} 9^(n-k) * binomial(3*k,k) * binomial(k/3,n-k)/(2*k+1).
PROG
(PARI) a(n) = sum(k=0, n, 9^(n-k)*binomial(3*k, k)*binomial(k/3, n-k)/(2*k+1));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 20 2024
STATUS
approved