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 A012258 #6 May 29 2019 20:00:10
%S A012258 1,1,1,4,13,84,469,4152,32985,370128,3818601,51861888,658371429,
%T A012258 10494283968,158197312701,2894815734912,50529852678321,
%U A012258 1043916757274880,20707179214389201,476720372375608320
%N A012258 Expansion of e.g.f.: exp(arctanh(arcsin(x)))=1+x+1/2!*x^2+4/3!*x^3+13/4!*x^4+84/5!*x^5...
%t A012258 With[{nn=20},CoefficientList[Series[Exp[ArcTanh[ArcSin[x]]],{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, May 29 2019 *)
%K A012258 nonn
%O A012258 0,4
%A A012258 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A012258 Definition clarified by _Harvey P. Dale_, May 29 2019