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 A024265 #12 Dec 21 2017 16:07:12
%S A024265 0,1,4,168,7808,725120,92516352,16783492096,3985986666496,
%T A024265 1212492098764800,457766291434373120,210284967226818953216,
%U A024265 115422873756041723510784,74614491201111668354449408
%N A024265 Expansion of tan(tan(x) * x)/2.
%F A024265 a(n) = A009707(n)/2. - _Vaclav Kotesovec_, Dec 21 2017
%F A024265 a(n) ~ c * d^n * n^(2*n + 1/2) / exp(2*n), where d = 3.9786913954409425781217887822690623430980810... is the root of the equation tan(2/sqrt(d)) = Pi*sqrt(d)/4 and c = 0.7028591997322... - _Vaclav Kotesovec_, Dec 21 2017
%t A024265 With[{nn=30},Take[CoefficientList[Series[(Tan[Tan[x]*x])/2,{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Nov 03 2015 *)
%Y A024265 Cf. A009707.
%K A024265 nonn
%O A024265 0,3
%A A024265 _R. H. Hardin_
%E A024265 Extended and signs tested 03/97.
%E A024265 Definition clarified and prior Mathematica program replaced by _Harvey P. Dale_, Nov 03 2015