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 A120617 #25 Mar 19 2023 14:15:12
%S A120617 1,-2,-4,8,16,-32,-64,128,256,-512,-1024,2048,4096,-8192,-16384,32768,
%T A120617 65536,-131072,-262144,524288,1048576,-2097152,-4194304,8388608,
%U A120617 16777216,-33554432,-67108864,134217728,268435456,-536870912,-1073741824,2147483648,4294967296,-8589934592
%N A120617 Hankel transform of g.f. 1/sqrt(1+4x^2).
%C A120617 Hankel transform of e.g.f. Bessel_I(0,2*sqrt(-1)*x) or (1,0,-2,0,6,0,-20,...).
%C A120617 Hankel transform of Sum{k=0..n} (-1)^(n-k)*C(n,k)^2.
%C A120617 Hankel transform of A098331.
%C A120617 Hankel transform of A082590. - _Paul Barry_, Apr 26 2009
%H A120617 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,-4).
%F A120617 G.f.: (1-2*x)/(1+4*x^2); a(n) = 2^n*(cos(Pi*(n+1)/2)+sin(Pi*(n+1)/2)).
%F A120617 a(0)=1, a(1)=-2, a(n)=-4*a(n-2). - _Harvey P. Dale_, Oct 12 2011
%F A120617 a(n) = ( 2*i^(n+1) )^n, where i=sqrt(-1). - _Bruno Berselli_, Oct 12 2011
%F A120617 E.g.f.: cos(2*x) - sin(2*x). - _Arkadiusz Wesolowski_, Aug 31 2012
%t A120617 LinearRecurrence[{0,-4},{1,-2},40] (* or *) CoefficientList[ Series[ (1-2x)/(1+4x^2),{x,0,40}],x] (* _Harvey P. Dale_, Oct 12 2011 *)
%Y A120617 Cf. A082590, A098331.
%K A120617 sign,easy
%O A120617 0,2
%A A120617 _Paul Barry_, Jun 17 2006