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 A077863 #21 Jan 05 2023 10:18:32
%S A077863 1,2,5,12,27,62,141,320,727,1650,3745,8500,19291,43782,99365,225512,
%T A077863 511807,1161562,2636201,5982940,13578467,30816750,69939565,158730000,
%U A077863 360242631,817581762,1855527025,4211175812,9557393387,21690799062,49227937461,111724322360
%N A077863 Expansion of (1-x)^(-1)/(1-x-2*x^2-2*x^3).
%H A077863 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,0,-2).
%F A077863 a(n) = a(n-1) + 2*a(n-2) + 2*a(n-3) + 1. - _Christian Krause_, Jan 02 2023
%t A077863 CoefficientList[Series[(1-x)^(-1)/(1-x-2x^2-2x^3),{x,0,40}],x] (* or *) LinearRecurrence[{2,1,0,-2},{1,2,5,12},40] (* _Harvey P. Dale_, Sep 14 2016 *)
%o A077863 (PARI) my(x='x+O('x^40)); Vec((1-x)^(-1)/(1-x-2*x^2-2*x^3)) \\ _Christian Krause_, Jan 02 2023
%Y A077863 Cf. A077946 (first differences).
%Y A077863 Cf. A078006 (second differences).
%K A077863 nonn,easy
%O A077863 0,2
%A A077863 _N. J. A. Sloane_, Nov 17 2002