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 A131756 #19 Dec 12 2023 09:13:56
%S A131756 2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,
%T A131756 3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,
%U A131756 -1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3,2,-1,3
%N A131756 Period 3: repeat [2, -1, 3].
%H A131756 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,1).
%F A131756 a(n) = 4/3+2/3*cos(2/3*Pi*n)-4/3*3^(1/2)*sin(2/3*Pi*n). - _R. J. Mathar_, Nov 15 2007
%F A131756 G.f.: (2-x+3*x^2)/(1-x^3). - _Jaume Oliver Lafont_, Mar 24 2009
%F A131756 a(n) = a(n-3) for n>2. - _Wesley Ivan Hurt_, Jul 01 2016
%p A131756 seq(op([2, -1, 3]), n=0..50); # _Wesley Ivan Hurt_, Jul 01 2016
%t A131756 PadRight[{}, 100, {2, -1, 3}] (* _Wesley Ivan Hurt_, Jul 01 2016 *)
%o A131756 (PARI) a(n)=[2,-1,3][1+n%3] \\ _Jaume Oliver Lafont_, Mar 24 2009
%o A131756 (Magma) &cat [[2, -1, 3]^^30]; // _Wesley Ivan Hurt_, Jul 01 2016
%Y A131756 Cf. A130784.
%K A131756 sign,easy
%O A131756 0,1
%A A131756 _Paul Curtz_, Oct 04 2007