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 A335848 #4 Jun 26 2020 23:44:46
%S A335848 1,1,4,21,152,1355,14568,182427,2612224,42073209,752981280,
%T A335848 14823367845,318347145216,7406554353939,185573713100160,
%U A335848 4981725842622795,142650055922872320,4340032650657965745,139809806502181765632,4754045863586538697077,170163141506896128122880
%N A335848 a(0) = 1; a(n) = Sum_{k=1..n} binomial(n,k) * k!! * a(n-k).
%F A335848 E.g.f.: 1 / (1 - x * exp(x^2/2) * (1 + sqrt(Pi/2) * erf(x/sqrt(2)))), where erf() is the error function.
%t A335848 a[0] = 1; a[n_] := a[n] = Sum[Binomial[n, k] k!! a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 20}]
%t A335848 nmax = 20; CoefficientList[Series[1/(1 - x Exp[x^2/2] (1 + Sqrt[Pi/2] Erf[x/Sqrt[2]])), {x, 0, nmax}], x] Range[0, nmax]!
%Y A335848 Cf. A002866, A006882, A292778, A308939.
%K A335848 nonn
%O A335848 0,3
%A A335848 _Ilya Gutkovskiy_, Jun 26 2020