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 A009273 #30 Apr 02 2025 17:32:46
%S A009273 1,2,4,-24,400,-5600,-103872,26975872,-3438685952,417995260416,
%T A009273 -51382607559680,5994623640856576,-454930757753597952,
%U A009273 -94991612229069430784,81515752167646959124480,-41079088828539119883878400,18870487103065970636941754368,-8553231336572387307575081566208
%N A009273 Expansion of e.g.f. exp(x*tanh(x)) (even powers only).
%H A009273 Vaclav Kotesovec, <a href="/A009273/b009273.txt">Table of n, a(n) for n = 0..240</a>
%F A009273 a(n) = Sum_(m=0..2*n, binomial(2*n,m)*Sum_(k=0..2*n-2*m, binomial(k+m-1,m-1)*(k+m)!*(-1)^(k)*2^(2*n-2*m-k)*stirling2(2*n-m,k+m))), n>0, a(0)=1. - _Vladimir Kruchinin_, Jun 06 2011
%t A009273 nmax = 20; Table[(CoefficientList[Series[E^(x*Tanh[x]), {x, 0, 2*nmax}], x]*Range[0, 2*nmax]!)[[k]], {k, 1, 2*nmax, 2}] (* _Vaclav Kotesovec_, May 24 2022 *)
%t A009273 With[{nn=40},Take[CoefficientList[Series[Exp[x Tanh[x]],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Apr 02 2025 *)
%o A009273 (Maxima)
%o A009273 a(n):=sum(binomial(2*n,m)*sum(binomial(k+m-1,m-1)*(k+m)!*(-1)^(k)*2^(2*n-2*m-k)*stirling2(2*n-m,k+m),k,0,2*n-2*m),m,0,2*n); /* _Vladimir Kruchinin_, Jun 06 2011 */
%Y A009273 Cf. A354245, A354399.
%K A009273 sign
%O A009273 0,2
%A A009273 _R. H. Hardin_
%E A009273 Extended with signs by _Olivier Gérard_, Mar 15 1997
%E A009273 More terms from _Vaclav Kotesovec_, May 25 2022