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 A009753 #26 Feb 13 2018 03:12:34
%S A009753 0,1,-2,8,-32,176,-1056,7664,-61312,559744,-5597440,61925632,
%T A009753 -743107584,9682766848,-135558735872,2035284795392,-32564556726272,
%U A009753 553807329689600,-9968531934412800,189431195638956032,-3788623912779120640
%N A009753 Expansion of tan(x)/(1+x).
%H A009753 G. C. Greubel, <a href="/A009753/b009753.txt">Table of n, a(n) for n = 0..445</a>
%F A009753 a(n) ~ n! * (-1)^(n+1) * tan(1). - _Vaclav Kotesovec_, Jan 24 2015
%p A009753 G(x):= tan(x)/(1+x): f[0]:=G(x): for n from 1 to 21 do f[n]:=diff(f[n-1],x) od: x:=0: seq(f[n],n=0..20); # _Zerinvary Lajos_, Apr 03 2009
%t A009753 CoefficientList[Series[Tan[x]/(1 + x), {x, 0, 20}], x] * Range[0, 20]! (* _Vaclav Kotesovec_, Jan 24 2015 *)
%o A009753 (PARI) x='x+O('x^30); concat([0], Vec(serlaplace(tan(x)/(1+x)))) \\ _G. C. Greubel_, Feb 12 2018
%Y A009753 Cf. A049471 (tan(1)).
%K A009753 sign,easy
%O A009753 0,3
%A A009753 _R. H. Hardin_
%E A009753 Extended with signs by _Olivier Gérard_, Mar 15 1997