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 A009010 #29 Jul 21 2018 20:26:28
%S A009010 1,1,13,397,22265,1996569,262056837,47378857957,11289999097969,
%T A009010 3429209143916337,1293273763150662781,592937704157794933821,
%U A009010 324791587492604492427881,209490216975221386279672393,157153880464155360205476452597
%N A009010 Expansion of e.g.f.: 1/cos(tan(x)) (even-indexed coefficients only).
%H A009010 G. C. Greubel, <a href="/A009010/b009010.txt">Table of n, a(n) for n = 0..200</a> (terms 0..50 from Vincenzo Librandi)
%F A009010 a(n) ~ (2*n)! * 8 / ((4+Pi^2) * (arctan(Pi/2))^(2*n+1)). - _Vaclav Kotesovec_, Jan 22 2015
%t A009010 f[x_] := Sec@Tan[x]; Table[Derivative[2*n][f][0], {n, 0, 14}] (* _Arkadiusz Wesolowski_, Aug 18 2012 *)
%t A009010 nn = 20; Table[(CoefficientList[Series[Sec[Tan[x]], {x, 0, 2*nn}], x] * Range[0, 2*nn]!)[[n]], {n, 1, 2*nn+1, 2}] (* _Vaclav Kotesovec_, Jan 22 2015 *)
%o A009010 (PARI) x='x+O('x^50); v=Vec(serlaplace(1/cos(tan(x)))); vector(#v\2,n,v[2*n-1]) \\ _G. C. Greubel_, Jul 21 2018
%K A009010 nonn
%O A009010 0,3
%A A009010 _R. H. Hardin_
%E A009010 Extended and signs tested by _Olivier Gérard_, Mar 15 1997
%E A009010 a(14) from _Arkadiusz Wesolowski_, Aug 18 2012