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 A077902 #12 May 29 2018 13:57:39
%S A077902 1,0,2,-2,5,-8,16,-28,53,-96,178,-326,601,-1104,2032,-3736,6873,
%T A077902 -12640,23250,-42762,78653,-144664,266080,-489396,900141,-1655616,
%U A077902 3045154,-5600910,10301681,-18947744,34850336,-64099760,117897841,-216847936,398845538,-733591314,1349284789,-2481721640
%N A077902 Expansion of (1-x)^(-1)/(1 + x - x^2 + x^3).
%H A077902 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,-2,1)
%t A077902 a=0; b=0; c=0; lst={}; Do[z=a+b+c+1; AppendTo[lst,z]; a=b; b=c; c=z; z=a+b+c-1; AppendTo[lst,z]; a=b; b=c; c=z,{n,5!}]; lst(*Abs values*) (* _Vladimir Joseph Stephan Orlovsky_, Feb 17 2010 *)
%o A077902 (PARI) Vec((1-x)^(-1)/(1+x-x^2+x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 26 2012
%K A077902 sign,easy
%O A077902 0,3
%A A077902 _N. J. A. Sloane_, Nov 17 2002