This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A372137 #8 Apr 20 2024 10:27:50
%S A372137 1,1,6,21,181,771,7728,34689,385632,1732971,21041598,92147697,
%T A372137 1217109951,5099210686,73380609681,289623783084,4564472639880,
%U A372137 16722146775195,290985244619874,974044248064611,18925364858562927,56848541164586820,1251693011560795635
%N A372137 G.f. A(x) satisfies A(x) = 1/( 1 - x * (1 + 9*x)^(1/3) * A(x)^2 ).
%F A372137 a(n) = Sum_{k=0..n} 9^(n-k) * binomial(3*k,k) * binomial(k/3,n-k)/(2*k+1).
%o A372137 (PARI) a(n) = sum(k=0, n, 9^(n-k)*binomial(3*k, k)*binomial(k/3, n-k)/(2*k+1));
%Y A372137 Cf. A364475, A372139.
%Y A372137 Cf. A372124.
%K A372137 nonn
%O A372137 0,3
%A A372137 _Seiichi Manyama_, Apr 20 2024