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 A009015 #24 Apr 16 2025 09:01:37
%S A009015 1,-1,13,-181,3865,-140521,6324517,-344747677,23853473329,
%T A009015 -1996865965009,193406280000061,-21615227339380357,
%U A009015 2778071540350106953,-403985610499148666041,65635628800688339178325,-11851572489741709802698861,2366329597668159364674862177
%N A009015 Expansion of e.g.f.: cos(x*cos(x)) (even powers only).
%H A009015 G. C. Greubel, <a href="/A009015/b009015.txt">Table of n, a(n) for n = 0..250</a>
%F A009015 a(n) = (-1)^n + (-1)^n*Sum_{j=0..(2*n-1)/2} binomial(2*n,2*j)*(Sum_{i=0..(2*j-1)/2} (j-i)^(2*n-2*j)*binomial(2*j,i))/(2^(4*j-2*n-1)). - _Vladimir Kruchinin_, Jun 06 2011
%p A009015 seq(coeff(series(factorial(n)*(cos(x*cos(x))), x,n+1),x,n),n=0..30,2); # _Muniru A Asiru_, Jul 21 2018
%t A009015 With[{nmax = 60}, CoefficientList[Series[Cos[x*Cos[x]], {x, 0, nmax}], x]*Range[0, nmax]!][[1 ;; -1 ;; 2]] (* _G. C. Greubel_, Jul 21 2018 *)
%o A009015 (Maxima)
%o A009015 a(n):=sum(binomial(2*n,2*j)*((sum((j-i)^(2*n-2*j)*binomial(2*j,i),i,0,((2*j-1)/2)))*(-1)^(n))/(2^(4*j-2*n-1)),j,0,(2*n-1)/2)+(-1)^n; /* _Vladimir Kruchinin_, Jun 06 2011 */
%o A009015 (PARI) my(x='x+O('x^50)); v=Vec(serlaplace(cos(x*cos(x)))); vector(#v\2,n,v[2*n-1]) \\ _G. C. Greubel_, Jul 21 2018
%K A009015 sign
%O A009015 0,3
%A A009015 _R. H. Hardin_
%E A009015 Extended with signs by _Olivier Gérard_, Mar 15 1997