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