OFFSET
0,2
FORMULA
a(n) = (1/(n+1)) * Sum_{k=0..floor(n/3)} binomial(n+k,k) * binomial(3*n-k+3,n-3*k).
PROG
(PARI) a(n) = sum(k=0, n\3, binomial(n+k, k)*binomial(3*n-k+3, n-3*k))/(n+1);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Sep 29 2023
STATUS
approved