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 A135431 #17 Feb 28 2021 11:10:11
%S A135431 0,1,2,3,6,10,17,30,51,88,152,261,450,775,1334,2298,3957,6814,11735,
%T A135431 20208,34800,59929,103202,177723,306054,527050,907625,1563006,2691627,
%U A135431 4635208,7982216,13746045,23671842,40764895,70200566,120891258,208184877,358511806
%N A135431 a(n) = a(n-1) + a(n-2) + a(n-3) - a(n-4) with a(0)=0, a(1)=1, a(2)=2 and a(3)=3.
%H A135431 G. C. Greubel, <a href="/A135431/b135431.txt">Table of n, a(n) for n = 0..1000</a>
%H A135431 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1, 1, 1, -1).
%F A135431 From _R. J. Mathar_, Mar 31 2008: (Start)
%F A135431 O.g.f.: x*(1+x)/(1-x-x^2-x^3+x^4).
%F A135431 a(n) = A116732(n+1) + A116732(n+2). (End)
%t A135431 LinearRecurrence[{1, 1, 1, -1}, {0, 1, 2, 3}, 40] (* _Vladimir Joseph Stephan Orlovsky_, Feb 03 2012 *)
%t A135431 nxt[{a_,b_,c_,d_}]:={b,c,d,d+c+b-a}; NestList[nxt,{0,1,2,3},40][[All,1]] (* _Harvey P. Dale_, Feb 28 2021 *)
%K A135431 nonn
%O A135431 0,3
%A A135431 _Mohamed Bouhamida_, Dec 13 2007