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