OFFSET
0,3
FORMULA
a(n) = (1/(n+1)) * Sum_{k=0..n} (-1)^k * binomial(n+1,k) * binomial(3*k,n-k).
PROG
(PARI) a(n) = sum(k=0, n, (-1)^k*binomial(n+1, k)*binomial(3*k, n-k))/(n+1);
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Aug 05 2023
STATUS
approved