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 A002020 M3876 N1588 #17 Feb 04 2022 00:35:41
%S A002020 1,1,1,1,-5,-17,83,593,-2893,-36101,172195,3421285,-15520165,
%T A002020 -467129785,1954015955,86971636825,-323371713725,-21196564551725,
%U A002020 66760541581475,6552909294409325,-16279195926455125,-2506384727801998625,4330877561309153875
%N A002020 a(n+1) = a(n) - n*(n-1)*a(n-1), with a(n) = 1 for n <= 3.
%D A002020 R. Kelisky, The numbers generated by exp(arctan x), Duke Math. J., 26 (1959), 569-581.
%D A002020 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A002020 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A002020 T. D. Noe, <a href="/A002020/b002020.txt">Table of n, a(n) for n = 0..100</a>
%p A002020 f := proc(n) option remember; if n<=3 then 1 else f(n-1)-(n-1)*(n-2)*f(n-2); fi; end;
%t A002020 t = {1, 1, 1, 1}; Do[AppendTo[t, t[[-1]] - (n-1) (n - 2) t[[-2]]], {n, 4, 20}] (* _T. D. Noe_, Aug 30 2012 *)
%K A002020 sign,easy
%O A002020 0,5
%A A002020 _N. J. A. Sloane_