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 A137717 #9 Oct 05 2017 17:02:40
%S A137717 1,-4,4,8,-32,32,64,-256,256,512,-2048,2048,4096,-16384,16384,32768,
%T A137717 -131072,131072,262144,-1048576,1048576,2097152,-8388608,8388608,
%U A137717 16777216,-67108864,67108864,134217728,-536870912,536870912
%N A137717 Hankel transform of A106191.
%C A137717 Hankel transform of A132310. [From _Paul Barry_, Apr 26 2009]
%H A137717 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (-2,-4).
%F A137717 G.f.: (1-2x)/(1+2x+4x^2).
%F A137717 a(n)=Product{k=0..n, (3*cos(2*pi*(k-1)/3)/2-5/4-2*0^k)^(n-k)};
%F A137717 a(n) = 2^n*A061347(n+2) = -2a(n-1)-4a(n-2). - _R. J. Mathar_, Feb 21 2008
%t A137717 LinearRecurrence[{-2,-4},{1,-4},30] (* _Harvey P. Dale_, Oct 05 2017 *)
%Y A137717 Apart from signs, essentially the same as A096252.
%K A137717 sign
%O A137717 0,2
%A A137717 _Paul Barry_, Feb 08 2008