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 A166249 #12 May 09 2016 04:37:46
%S A166249 1,1,0,2,5,12,22,43,84,170,341,684,1366,2731,5460,10922,21845,43692,
%T A166249 87382,174763,349524,699050,1398101,2796204,5592406,11184811,22369620,
%U A166249 44739242,89478485,178956972,357913942,715827883,1431655764,2863311530
%N A166249 a(n) = 2*a(n-1) - a(n-3) + 2*a(n-4) for n>3, a(0)=a(1)=1, a(2)=0, a(3)=2.
%H A166249 G. C. Greubel, <a href="/A166249/b166249.txt">Table of n, a(n) for n = 0..1000</a>
%H A166249 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,0,-1,2).
%F A166249 From _R. J. Mathar_, Nov 09 2009: (Start)
%F A166249 G.f.: (1-x+3*x^3-2*x^2)/((1-2*x)*(1+x)*(x^2-x+1)).
%F A166249 a(n) = A001045(n) + A010892(n+1). (End)
%t A166249 LinearRecurrence[{2, 0, -1, 2}, {1, 1, 0, 2}, 100] (* _G. C. Greubel_, May 08 2016 *)
%K A166249 nonn,easy
%O A166249 0,4
%A A166249 _Paul Curtz_, Oct 10 2009
%E A166249 Edited by _N. J. A. Sloane_, Oct 13 2009
%E A166249 Extended by _R. J. Mathar_, Nov 09 2009