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 A335981 #7 Jul 04 2020 01:44:23
%S A335981 1,4,13,31,40,-23,-95,490,823,-8393,3766,174775,-658787,-2751404,
%T A335981 34033297,-55552037,-1170734432,9362348365,3277050925,-562286419646,
%U A335981 3848880970147,8815342530739,-356804325202730,2389771436686339,8677476137729929,-302470260552857660
%N A335981 Expansion of e.g.f. exp(3 * (1 - exp(-x)) + x).
%F A335981 a(n) = exp(3) * (-1)^n * Sum_{k>=0} (-3)^k * (k - 1)^n / k!.
%F A335981 a(0) = 1; a(n) = a(n-1) + 3 * Sum_{k=0..n-1} (-1)^(n-k-1) * binomial(n-1,k) * a(k).
%t A335981 nmax = 25; CoefficientList[Series[Exp[3 (1 - Exp[-x]) + x], {x, 0, nmax}], x] Range[0, nmax]!
%t A335981 a[0] = 1; a[n_] := a[n] = a[n - 1] + 3 Sum[(-1)^(n - k - 1) Binomial[n - 1, k] a[k], {k, 0, n - 1}]; Table[a[n], {n, 0, 25}]
%Y A335981 Cf. A027710, A078940, A109747, A309084, A335868, A335980, A335982.
%K A335981 sign
%O A335981 0,2
%A A335981 _Ilya Gutkovskiy_, Jul 03 2020