OFFSET
0,4
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, (-1)^k*binomial(3*k, n-k)*binomial(2*k, k)/(k+1));
(PARI) my(N=40, x='x+O('x^N)); Vec(2/(1+sqrt(1+4*x*(1+x)^3)))
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jan 25 2023
STATUS
approved