OFFSET
0,2
FORMULA
a(n) = (-1)^(n-1) * Sum_{k=0..n} binomial(3*k-1,k) * binomial(3*(2*k-1),n-k)/(3*k-1).
PROG
(PARI) a(n) = (-1)^(n-1)*sum(k=0, n, binomial(3*k-1, k)*binomial(3*(2*k-1), n-k)/(3*k-1));
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Oct 18 2023
STATUS
approved