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 A009391 #18 Oct 14 2022 11:59:07
%S A009391 0,1,-1,3,-4,25,-61,427,-1184,12465,-49201,555731,-2361844,35135945,
%T A009391 -191422141,2990414715,-17147588384,329655706465,-2289437638081,
%U A009391 45692713833379,-329955144475204,7777794952988025,-65643617893832221
%N A009391 Expansion of log(1 + tanh(x))/cos(x).
%F A009391 a(n) ~ n! * 2^(n+1) * (Pi + (1+(-1)^n) * log(2/(1+exp(Pi)))) / Pi^(n+1). - _Vaclav Kotesovec_, Jan 24 2015
%t A009391 CoefficientList[Series[Log[1 + Tanh[x]]*Sec[x], {x, 0, 20}], x] * Range[0, 20]! (* _Vaclav Kotesovec_, Jan 24 2015 *)
%Y A009391 a(2n+1) = A009843(n), |a(2n)| = A009391(2n).
%K A009391 sign,easy
%O A009391 0,4
%A A009391 _R. H. Hardin_
%E A009391 Extended with signs by _Olivier Gérard_, Mar 15 1997
%E A009391 Incorrect Mathematica program deleted by _Harvey P. Dale_, Oct 14 2022