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 A257970 #40 Jun 29 2023 13:20:15
%S A257970 1,2,5,16,48,144,432,1296,3888,11664,34992,104976,314928,944784,
%T A257970 2834352,8503056,25509168,76527504,229582512,688747536,2066242608,
%U A257970 6198727824,18596183472,55788550416,167365651248,502096953744,1506290861232,4518872583696,13556617751088
%N A257970 a(1) = 1, a(2) = 2, a(3) = 5; thereafter a(n) = 2 * Sum_{k=1..n-1} a(k).
%H A257970 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (3).
%F A257970 G.f.: -x*(x+1)*(x-1)^2/(3*x-1). [Corrected by _Vincenzo Librandi_, May 25 2015]
%F A257970 a(n) = 3*a(n-1) for n>=5.
%F A257970 a(n) = 16*3^(n-4) for n>=4.
%t A257970 CoefficientList[Series[(x + 1) (x - 1)^2/(1 - 3 x), {x, 0, 33}], x] (* _Vincenzo Librandi_, May 25 2015 *)
%t A257970 Join[{1,2,5},NestList[3#&,16,30]] (* _Harvey P. Dale_, Dec 20 2018 *)
%o A257970 (Magma) [1,2,5] cat [16*3^(n-4): n in [4..50]]; // _Vincenzo Librandi_, May 25 2015
%K A257970 nonn,easy,less
%O A257970 1,2
%A A257970 _Harry Farrar_, May 15 2015