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 A296941 #5 Dec 26 2017 18:06:10
%S A296941 0,1,1,18,227,12125,542448,55071205,5492843269,905996551626,
%T A296941 159770279801855,39299019878991521,10721872262093222016,
%U A296941 3707660329253983397113,1438816154956071399594457,668949924061617421125859650,348908555505788456739965412203
%N A296941 Expansion of e.g.f. arcsin(x*tan(x/2)) (even powers only).
%F A296941 a(n) = (2*n)! * [x^(2*n)] arcsin(x*tan(x/2)).
%e A296941 arcsin(x*tan(x/2)) = x^2/2! + x^4/4! + 18*x^6/6! + 227*x^8/8! + ...
%t A296941 nmax = 16; Table[(CoefficientList[Series[ArcSin[x Tan[x/2]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
%Y A296941 Cf. A001469, A001818, A012780, A110501, A296839, A296841, A296842, A296853, A296854, A296856, A296939, A296940, A296942.
%K A296941 nonn
%O A296941 0,4
%A A296941 _Ilya Gutkovskiy_, Dec 22 2017