%I #25 Oct 04 2023 12:50:57

%S 1,1,6,33,209,1425,10206,75751,577494,4495368,35582439,285524184,

%T 2317387098,18990744137,156918815760,1305927563487,10936673012579,

%U 92098612059051,779391530714589,6624730079900931,56532669993156696,484156547579505717,4159926573597719575

%N G.f. A(x) satisfies A(x) = 1 + x*(1 + x)^3*A(x)^3.

%F a(n) = Sum_{k=0..n} binomial(3*k,n-k) * binomial(3*k,k)/(2*k+1).

%o (PARI) a(n) = sum(k=0, n, binomial(3*k, n-k)*binomial(3*k, k)/(2*k+1));

%Y Cf. A073155, A366216.

%K nonn

%O 0,3

%A _Seiichi Manyama_, Oct 04 2023