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 A136336 #12 Jan 14 2019 18:04:23
%S A136336 1,2,4,7,15,27,59,107,235,427,939,1707,3755,6827,15019,27307,60075,
%T A136336 109227,240299,436907,961195,1747627,3844779,6990507,15379115,
%U A136336 27962027,61516459,111848107,246065835,447392427,984263339,1789569707,3937053355,7158278827
%N A136336 a(n) = a(n-1) + 4*a(n-2) - 4*a(n-3) for n>3.
%H A136336 Colin Barker, <a href="/A136336/b136336.txt">Table of n, a(n) for n = 0..1000</a>
%H A136336 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,4,-4).
%F A136336 From _Colin Barker_, Mar 17 2017: (Start)
%F A136336 G.f.: (1 + x - 2*x^2 - x^3) / ((1 - x)*(1 - 2*x)*(1 + 2*x)).
%F A136336 a(n) = (8 + (-2)^n + 21*2^n) / 24 for n>0.
%F A136336 (End)
%t A136336 LinearRecurrence[{1,4,-4},{1,2,4,7},50] (* _Harvey P. Dale_, Jan 14 2019 *)
%o A136336 (PARI) Vec((1 + x - 2*x^2 - x^3) / ((1 - x)*(1 - 2*x)*(1 + 2*x)) + O(x^50)) \\ _Colin Barker_, Mar 17 2017
%K A136336 nonn,easy
%O A136336 0,2
%A A136336 _Paul Curtz_, Mar 28 2008