OFFSET
0,2
FORMULA
a(n) = [x^n] 1/((1-x+x^2) * (1-x)^(2*n)).
PROG
(PARI) a(n) = sum(k=0, n\2, (-1)^k*binomial(3*n-k, n-2*k));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 06 2024
STATUS
approved