OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..n} binomial(n+k-1,k) * binomial(3*k,n-k).
a(n) = [x^n] 1/(1 - x*(1+x)^3)^n.
PROG
(PARI) a(n) = sum(k=0, n, binomial(n+k-1, k)*binomial(3*k, n-k));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 08 2023
STATUS
approved