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 A077936 #36 Sep 08 2022 08:45:07
%S A077936 1,2,6,17,48,136,385,1090,3086,8737,24736,70032,198273,561346,1589270,
%T A077936 4499505,12738896,36066072,102109441,289089922,818464798,2317218881,
%U A077936 6560457280,18573817120,52585767681,148879626882,421504606246,1193354233937,3378597307248
%N A077936 Expansion of 1/(1 - 2*x - 2*x^2 - x^3).
%C A077936 Row sum of A158687. - _Paul Barry_, Mar 24 2009
%H A077936 Vincenzo Librandi, <a href="/A077936/b077936.txt">Table of n, a(n) for n = 0..500</a>
%H A077936 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,2,1).
%F A077936 a(n) = Sum_{k=0..n} Sum_{j=0..n-k} C(n-j,k)*C(2k,j). - _Paul Barry_, Mar 24 2009
%F A077936 a(n) = 2*a(n-1) + 2*a(n-2) + a(n-3) with a(0) = 1, a(1) = 2, a(2) = 6. - _Taras Goy_, Aug 04 2017
%t A077936 CoefficientList[Series[1/(1-2*x-2*x^2-x^3),{x,0,40}],x] (* or *) LinearRecurrence[{2,2,1},{1,2,6},40] (* _Vladimir Joseph Stephan Orlovsky_, Jan 30 2012 *)
%o A077936 (PARI) Vec(1/(1-2*x-2*x^2-x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Jan 31 2012
%o A077936 (Magma) I:=[1,2,6]; [n le 3 select I[n] else 2*Self(n-1)+2*Self(n-2)+Self(n-3): n in [1..30]]; // _Vincenzo Librandi_, Jul 06 2015
%K A077936 nonn,easy
%O A077936 0,2
%A A077936 _N. J. A. Sloane_, Nov 17 2002