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 A131505 #9 May 05 2021 09:41:50
%S A131505 0,-1,0,2,1,-1,1,4,2,-1,2,6,3,-1,3,8,4,-1,4,10,5,-1,5,12,6,-1,6,14,7,
%T A131505 -1,7,16,8,-1,8,18,9,-1,9,20,10,-1,10,22,11,-1,11,24,12,-1,12,26,13,
%U A131505 -1,13,28,14,-1,14,30,15,-1,15,32,16,-1,16,34,17,-1,17,36,18,-1,18,38,19,-1,19,40,20,-1,20,42,21,-1,21,44,22,-1,22,46,23
%N A131505 n, -1, n, 2n+2.
%C A131505 Based on A129819 (the first with 0 0).
%H A131505 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (0,-1,0,2,1,-1).
%F A131505 G.f.: (-x*(1 - 2*x + x^2 - x^3))/(1 - x + x^2 - x^3)^2. - _Georg Fischer_, May 05 2021
%p A131505 seq(coeff(series( (-x*(1 - 2*x + x^2 - x^3))/(1 - x + x^2 - x^3)^2, x, n+1), x, n), n = 0..40); # _Georg Fischer_, May 05 2021
%t A131505 Flatten[Table[{n,-1,n,2n+2},{n,0,30}]] (* _Harvey P. Dale_, Nov 15 2011 *)
%K A131505 sign,easy
%O A131505 0,4
%A A131505 _Paul Curtz_, Aug 13 2007