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 A106201 #10 May 22 2020 15:41:51
%S A106201 0,1,1,1,1,-3,-11,-23,-39,-43,-3,129,417,877,1349,1305,-407,-5627,
%T A106201 -16243,-32079,-46287,-37987,35285,236873,623609,1162293,1559837,
%U A106201 1009889,-2034495,-9728051,-23660955,-41633415,-51467895,-22390171,101331373,391586577,887713361,1473400829,1653762805,267778473,-4669059303,-15499500395
%N A106201 Expansion of Re(x/(1-x-2*i*x^2)), i=sqrt(-1).
%C A106201 Imaginary part is A106202.
%H A106201 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,0,-4)
%F A106201 G.f.: x*(1-x)/(1-2*x+x^2+4*x^4).
%F A106201 a(n) = sum(k=0..floor((n-1)/2), C(n-k-1, k)*2^k*cos(pi*k/2) ).
%t A106201 LinearRecurrence[{2,-1,0,-4},{0,1,1,1},50] (* _Harvey P. Dale_, May 22 2020 *)
%Y A106201 Cf. A104862, A014292.
%K A106201 easy,sign
%O A106201 0,6
%A A106201 _Paul Barry_, Apr 25 2005