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 A380768 #14 Feb 03 2025 11:07:44
%S A380768 1,2,19,361,10481,411961,20477185,1232420449,87148819441,
%T A380768 7083132622561,650681345267801,66674532650884753,7540078499903430937,
%U A380768 932840158873518067537,125332464410926005144241,18173310946391976757487041,2828702590649296770695135585,470432341506749952275419504321
%N A380768 E.g.f. A(x) satisfies A(x) = exp(x * A(x) / (1 - x*A(x)^2)) / (1 - x*A(x)^2).
%F A380768 a(n) = n! * Sum_{k=0..n} (2*n-k+1)^(k-1) * binomial(3*n-k,n-k)/k!.
%o A380768 (PARI) a(n, q=1, r=2, s=1, t=1, u=1) = q*n!*sum(k=0, n, (r*n+(s-r)*k+q)^(k-1)*binomial((r*u+1)*n+((s-r)*u+t-1)*k+q*u-1, n-k)/k!);
%Y A380768 Cf. A380726, A380769.
%Y A380768 Cf. A377888, A380771.
%K A380768 nonn
%O A380768 0,2
%A A380768 _Seiichi Manyama_, Feb 02 2025