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 A296940 #5 Dec 22 2017 10:20:57
%S A296940 1,0,-3,-15,406,14355,-189123,-42283696,-837846615,284972761557,
%T A296940 28521503291230,-3070544172379761,-1054107683427761463,
%U A296940 1143265731049052000,54900209444888714822181,7959249060310612253252265,-3679623847504649619798598778,-1631286181830482909037469295781
%N A296940 Expansion of e.g.f. sech(x*tan(x/2)) (even powers only).
%F A296940 a(n) = (2*n)! * [x^(2*n)] sech(x*tan(x/2)).
%e A296940 sech(x*tan(x/2)) = 1 - 3*x^4/4! - 15*x^6/6! + 406*x^8/8! + ...
%t A296940 nmax = 17; Table[(CoefficientList[Series[Sech[x Tan[x/2]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
%Y A296940 Cf. A000364, A001469, A009011, A110501, A296839, A296841, A296842, A296853, A296854, A296856, A296939.
%K A296940 sign
%O A296940 0,3
%A A296940 _Ilya Gutkovskiy_, Dec 22 2017