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 A012835 #6 Jul 30 2020 20:08:02
%S A012835 1,-1,25,-761,42065,-3898321,521329705,-96017526281,23356125062945,
%T A012835 -7240133620964641,2786977252450841785,-1304375789021987253401,
%U A012835 729394652578427432666225,-480281453260247282387992561
%N A012835 arctan(sec(x)*tanh(x))=x-1/3!*x^3+25/5!*x^5-761/7!*x^7+42065/9!*x^9...
%t A012835 With[{nn=30},Take[CoefficientList[Series[ArcTan[Sec[x]Tanh[x]],{x,0,nn}],x] Range[0,nn-1]!,{2,-1,2}]] (* _Harvey P. Dale_, Jul 30 2020 *)
%K A012835 sign
%O A012835 0,3
%A A012835 Patrick Demichel (patrick.demichel(AT)hp.com)