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 A133513 #20 Dec 12 2023 08:47:29
%S A133513 0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,
%T A133513 3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,
%U A133513 -1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3,0,1,-3,0,-1,3
%N A133513 Period 6: repeat [0, 1, -3, 0, -1, 3].
%H A133513 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,-1).
%F A133513 From _Wesley Ivan Hurt_, Jun 21 2016: (Start)
%F A133513 G.f.: x*(1-3*x)/(1+x^3).
%F A133513 a(n) + a(n-3) = 0 for n>2.
%F A133513 a(n) = 2 * sin(n*Pi/3) * (4*sin(2*n*Pi/3) - sqrt(3))/3. (End)
%p A133513 A133513:=n->[0, 1, -3, 0, -1, 3][(n mod 6)+1]: seq(A133513(n), n=0..100); # _Wesley Ivan Hurt_, Jun 21 2016
%t A133513 PadRight[{}, 100, {0, 1, -3, 0, -1, 3}] (* _Wesley Ivan Hurt_, Jun 21 2016 *)
%o A133513 (PARI) a(n)=[0,1,-3,0,-1,3][n%6+1] \\ _Charles R Greathouse IV_, Jun 02 2011
%o A133513 (Magma) &cat [[0, 1, -3, 0, -1, 3]^^20]; // _Wesley Ivan Hurt_, Jun 21 2016
%K A133513 sign,easy,less
%O A133513 0,3
%A A133513 _Paul Curtz_, Nov 30 2007