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 A380721 #9 Jan 31 2025 09:25:26
%S A380721 1,3,29,511,13313,462401,20140495,1056765711,64931273601,
%T A380721 4575023966017,363744086548751,32219262817769039,3146690718151835233,
%U A380721 335963164545043929921,38931639595489583488239,4866587415704561667715471,652773358729046023136421377,93523037570967777721191018881
%N A380721 E.g.f. A(x) satisfies A(x) = exp(x / (1 - x*A(x))^3) / (1 - x*A(x))^2.
%F A380721 a(n) = n! * Sum_{k=0..n} (n-k+1)^(k-1) * binomial(3*n+1,n-k)/k!.
%o A380721 (PARI) a(n) = n!*sum(k=0, n, (n-k+1)^(k-1)*binomial(3*n+1, n-k)/k!);
%Y A380721 Cf. A380665, A380722.
%K A380721 nonn
%O A380721 0,2
%A A380721 _Seiichi Manyama_, Jan 30 2025