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 A009118 #20 Jul 26 2018 03:21:56
%S A009118 1,-1,-11,-181,-3863,-66121,4478365,1211701763,226423491793,
%T A009118 43068302925551,8876725117679941,1997577117130009403,
%U A009118 483811389670392875449,121594250947356501211559,28960468994349845642813677
%N A009118 Expansion of e.g.f. cos(x/cos(x)) (even powers only).
%H A009118 G. C. Greubel, <a href="/A009118/b009118.txt">Table of n, a(n) for n = 0..240</a>
%F A009118 a(n) = 2*Sum_{m=1..n-1} binomial(2*n,2*m)*Sum_{j=0..n-m} binomial(m+j-1,j)*4^(n-m-j)*Sum_{i=0..j} (i-j)^(2*n-2*m)*binomial(2*j,i)*(-1)^(n+j-i) +(-1)^n. - _Vladimir Kruchinin_, Jun 28 2011
%t A009118 With[{nmax = 60}, CoefficientList[Series[Cos[x/Cos[x]], {x, 0, nmax}], x]*Range[0, nmax]!][[1 ;; -1 ;; 2]] (* _G. C. Greubel_, Jul 26 2018 *)
%o A009118 (Maxima)
%o A009118 a(n):=2*sum(binomial(2*n, 2*m)*sum(binomial(m+j-1, j)*4^(n-m-j)*sum((i-j)^(2*n-2*m)*binomial(2*j, i)*(-1)^(n+j-i), i, 0, j), j, 0, n-m), m, 1, n-1)+(-1)^n; /* _Vladimir Kruchinin_, Jun 28 2011 */
%o A009118 (PARI) x='x+O('x^50); v=Vec(serlaplace(cos(x/cos(x)))); vector(#v\2,n,v[2*n-1]) \\ _G. C. Greubel_, Jul 26 2018
%K A009118 sign
%O A009118 0,3
%A A009118 _R. H. Hardin_
%E A009118 Extended with signs by _Olivier Gérard_, Mar 15 1997