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 A296939 #5 Dec 22 2017 09:01:01
%S A296939 1,0,3,15,644,17145,1124673,74115496,7730031915,921044459943,
%T A296939 145334164141820,26830525240048761,6053646614467427553,
%U A296939 1586816790903080698000,487642998132913180824819,171640559783810345998524735,69078935661419038650738789428
%N A296939 Expansion of e.g.f. sec(x*tan(x/2)) (even powers only).
%F A296939 a(n) = (2*n)! * [x^(2*n)] sec(x*tan(x/2)).
%e A296939 sec(x*tan(x/2)) = 1 + 3*x^4/4! + 15*x^6/6! + 644*x^8/8! + ...
%t A296939 nmax = 16; Table[(CoefficientList[Series[Sec[x Tan[x/2]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
%Y A296939 Cf. A000364, A001469, A009010, A110501, A296839, A296841, A296842, A296853, A296854, A296856, A296940.
%K A296939 nonn
%O A296939 0,3
%A A296939 _Ilya Gutkovskiy_, Dec 22 2017