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 A336959 #5 Aug 10 2020 00:20:30
%S A336959 1,1,-2,-6,40,120,-1872,-3920,155776,56448,-19946240,44799744,
%T A336959 3588719616,-21265587200,-850126505984,9423227873280,251457224998912,
%U A336959 -4665150579572736,-88212028284665856,2663461772025462784,34353949630376181760,-1756678038088484388864
%N A336959 E.g.f.: 1 / (1 - x * exp(-2*x)).
%F A336959 a(n) = n! * Sum_{k=0..n} (-2 * (n-k))^k / k!.
%F A336959 a(0) = 1; a(n) = Sum_{k=1..n} binomial(n,k) * k * (-2)^(k-1) * a(n-k).
%t A336959 nmax = 21; CoefficientList[Series[1/(1 - x Exp[-2 x]), {x, 0, nmax}], x] Range[0, nmax]!
%t A336959 Join[{1}, Table[n! Sum[(-2 (n - k))^k/k!, {k, 0, n}], {n, 1, 21}]]
%t A336959 a[0] = 1; a[n_] := a[n] = Sum[Binomial[n, k] k (-2)^(k - 1) a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 21}]
%Y A336959 Cf. A001787, A155585, A302397, A336950, A336958.
%K A336959 sign
%O A336959 0,3
%A A336959 _Ilya Gutkovskiy_, Aug 09 2020