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 A343975 #18 Jun 07 2021 14:55:19
%S A343975 1,3,15,81,489,3237,23211,178707,1467051,12768345,117263829,
%T A343975 1131901521,11444383251,120847326879,1329303053391,15197269729689,
%U A343975 180211641841353,2212525627591533,28078380387448515,367782119667874083,4965441830591976339,69014083524412401873,986364827548578356421
%N A343975 a(0) = 1; a(n) = 3 * Sum_{k=1..n} binomial(n,k) * a(k-1).
%F A343975 G.f. A(x) satisfies: A(x) = 1 + 3 * x * A(x/(1 - x)) / (1 - x)^2.
%t A343975 a[0] = 1; a[n_] := a[n] = 3 Sum[Binomial[n, k] a[k - 1], {k, 1, n}]; Table[a[n], {n, 0, 22}]
%t A343975 nmax = 22; A[_] = 0; Do[A[x_] = 1 + 3 x A[x/(1 - x)]/(1 - x)^2 + O[x]^(nmax + 1) // Normal, nmax + 1]; CoefficientList[A[x], x]
%Y A343975 Cf. A027710, A032033, A040027, A078940, A343523, A344735, A344840, A345077, A345078, A345081.
%K A343975 nonn
%O A343975 0,2
%A A343975 _Ilya Gutkovskiy_, Jun 07 2021