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 A077849 #16 Nov 30 2017 16:48:10
%S A077849 1,3,8,21,54,138,352,897,2285,5820,14823,37752,96148,244872,623645,
%T A077849 1588311,4045140,10302237,26237926,66823230,170186624,433434405,
%U A077849 1103878665,2811378360,7160069791,18235396608,46442241368,118279949136,301237536249,767197263003
%N A077849 Expansion of (1-x)^(-1)/(1 - 2*x - x^2 - x^3).
%H A077849 I. M. Gessel, Ji Li, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL16/Gessel/gessel6.html">Compositions and Fibonacci identities</a>, J. Int. Seq. 16 (2013) 13.4.5
%H A077849 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (3,-1,0,-1)
%p A077849 A077939 := proc(n) if n< 0 then 0; else coeftayl( 1/(1-2*x-x^2-x^3) ,x=0,n) ; end if; end proc:
%p A077849 A077849 := proc(n) (-1+4*A077939(n)+2*A077939(n-1)+A077939(n-2))/3 ; end proc:
%p A077849 seq(A077849(n),n=0..20) ; # _R. J. Mathar_, Mar 22 2011
%t A077849 CoefficientList[Series[(1-x)^(-1)/(1-2x-x^2-x^3),{x,0,40}],x] (* or *) LinearRecurrence[{3,-1,0,-1},{1,3,8,21},40] (* _Harvey P. Dale_, Nov 01 2016 *)
%o A077849 (PARI) Vec((1-x)^(-1)/(1-2*x-x^2-x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 26 2012
%Y A077849 Partial sums of A077939.
%K A077849 nonn,easy
%O A077849 0,2
%A A077849 _N. J. A. Sloane_, Nov 17 2002