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 A012388 #7 Feb 08 2015 05:53:12
%S A012388 0,2,16,392,21376,2117792,333550336,76433892992,23981418704896,
%T A012388 9862199971860992,5144452992189485056,3317894031273853601792,
%U A012388 2591931617534673513250816,2411532576645611695219515392
%N A012388 arcsin(tan(x)*tan(x))=2/2!*x^2+16/4!*x^4+392/6!*x^6+21376/8!*x^8...
%F A012388 a(n) ~ 2^(4*n-1) * (2*n)! / (n^(3/2) * Pi^(2*n)). - _Vaclav Kotesovec_, Feb 08 2015
%t A012388 nn = 20; Table[(CoefficientList[Series[ArcSin[Tan[x]^2], {x, 0, 2*nn}], x] * Range[0, 2*nn]!)[[n]], {n, 1, 2*nn+1, 2}] (* _Vaclav Kotesovec_, Feb 08 2015 *)
%K A012388 nonn
%O A012388 0,2
%A A012388 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A012388 a(0)=0 prepended by _Vaclav Kotesovec_, Feb 08 2015