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 A012193 #6 Jun 06 2018 15:11:27
%S A012193 1,-1,17,-721,58337,-7657441,1482169457,-396854618161,140415423261377,
%T A012193 -63432792091634881,35619694647751942097,-24334169873267896659601,
%U A012193 19872330538404158435859617
%N A012193 sech(arctan(sin(x)))=1-1/2!*x^2+17/4!*x^4-721/6!*x^6+58337/8!*x^8...
%t A012193 With[{nn=30},Take[CoefficientList[Series[Sech[ArcTan[Sin[x]]],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Jun 06 2018 *)
%K A012193 sign
%O A012193 0,3
%A A012193 Patrick Demichel (patrick.demichel(AT)hp.com)