OFFSET
0,3
FORMULA
G.f. A(x) satisfies A(x) = 1/(1 - x * (1+x)^3 * A(x)).
G.f.: 2 / (1 + sqrt( 1 - 4*x*(1+x)^3 )).
PROG
(PARI) a(n) = sum(k=0, n, binomial(3*k, n-k)*binomial(2*k, k)/(k+1));
(PARI) my(N=30, x='x+O('x^N)); Vec(2/(1+sqrt(1-4*x*(1+x)^3)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jan 25 2023
STATUS
approved