OFFSET
0,2
FORMULA
a(n) = [x^n] 1/( 1/(1 + x) - x^3 )^(2*n).
a(n) == 0 (mod 2) for n>0.
PROG
(PARI) a(n) = sum(k=0, n\3, binomial(2*n+k-1, k)*binomial(2*n+k, n-3*k));
CROSSREFS
KEYWORD
nonn,new
AUTHOR
Seiichi Manyama, Dec 15 2024
STATUS
approved