OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..floor(n/3)} binomial(2*n-k+1,k) * binomial(3*n-4*k,n-3*k)/(2*n-k+1).
PROG
(PARI) a(n) = sum(k=0, n\3, binomial(2*n-k+1, k)*binomial(3*n-4*k, n-3*k)/(2*n-k+1));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 04 2023
STATUS
approved