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 A380726 #9 Jan 31 2025 09:25:42
%S A380726 1,2,23,526,18345,865426,51606511,3725086590,315869177777,
%T A380726 30781410753250,3390102419068071,416446509483046318,
%U A380726 56455962861401232025,8372599773137199223794,1348414830158700569758655,234364024637335981658563486,43725325359127416298442233569
%N A380726 E.g.f. A(x) satisfies A(x) = exp(x * A(x)^2 / (1 - x*A(x)^2)) / (1 - x*A(x)^2).
%F A380726 a(n) = n! * Sum_{k=0..n} (2*n+1)^(k-1) * binomial(3*n,n-k)/k!.
%o A380726 (PARI) a(n) = n!*sum(k=0, n, (2*n+1)^(k-1)*binomial(3*n, n-k)/k!);
%Y A380726 Cf. A380663, A380727.
%Y A380726 Cf. A380723.
%K A380726 nonn
%O A380726 0,2
%A A380726 _Seiichi Manyama_, Jan 31 2025