OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..floor(n/3)} (-1)^k * binomial(2*(n-3*k)+1,k) * binomial(3*(n-3*k),n-3*k)/(2*(n-3*k)+1).
PROG
(PARI) a(n) = sum(k=0, n\3, (-1)^k*binomial(2*(n-3*k)+1, k)*binomial(3*(n-3*k), n-3*k)/(2*(n-3*k)+1));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 03 2023
STATUS
approved