OFFSET
0,3
FORMULA
a(n) = (-1)^n * Sum_{k=0..n} 9^(n-k) * binomial(n,k) * binomial(k/3,n-k)/(n-k+1).
PROG
(PARI) a(n) = (-1)^n*sum(k=0, n, 9^(n-k)*binomial(n, k)*binomial(k/3, n-k)/(n-k+1));
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Apr 19 2024
STATUS
approved