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 A013453 #8 Feb 06 2015 15:40:06
%S A013453 0,4,0,544,35840,707584,298721280,26768052224,5564833464320,
%T A013453 1787224334270464,379247341296680960,181364573942845538304,
%U A013453 71776324876296729395200,35085662010847230106271744
%N A013453 tan(tan(x)-tanh(x))=4/3!*x^3+544/7!*x^7+35840/9!*x^9+707584/11!*x^11...
%F A013453 a(n) ~ 2 * (2*n+1)! / ((1/(cos(r))^2 - 1/(cosh(r))^2) * r^(2*n+2)), where r = 1.175536528296496445322181894766544798346231323714914157798... is the root of the equation tan(r)-tanh(r) = Pi/2. - _Vaclav Kotesovec_, Feb 06 2015
%t A013453 nn = 20; Table[(CoefficientList[Series[Tan[Tan[x] - Tanh[x]], {x, 0, 2*nn+1}], x] * Range[0, 2*nn+1]!)[[n]], {n, 2, 2*nn, 2}] (* _Vaclav Kotesovec_, Feb 06 2015 *)
%K A013453 nonn
%O A013453 0,2
%A A013453 Patrick Demichel (patrick.demichel(AT)hp.com)
%E A013453 a(0)=0 prepended by _Vaclav Kotesovec_, Feb 06 2015