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 A174132 #27 Sep 08 2022 08:45:51
%S A174132 3,5,19,53,163,485,1459,4373,13123,39365,118099,354293,1062883,
%T A174132 3188645,9565939,28697813,86093443,258280325,774840979,2324522933,
%U A174132 6973568803,20920706405,62762119219,188286357653,564859072963
%N A174132 2*3^(n-1)-(-1)^n.
%H A174132 Vincenzo Librandi, <a href="/A174132/b174132.txt">Table of n, a(n) for n = 1..200</a>
%H A174132 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (2,3).
%F A174132 From _Bruno Berselli_, Jan 28 2011 - Jan 30 2011: (Start)
%F A174132 G.f.: x*(3-x)/((1+x)*(1-3*x)).
%F A174132 a(n) = A062547(A042963(n-1)).
%F A174132 a(n) = 2*sum[a(i), i=1..n-1]-2*(-1)^n+1 for n>1. (End)
%t A174132 Table[(2 3^(n-1) - (-1)^n), {n, 30}] (* or *) CoefficientList[Series[(3 - x)/((1 + x)(1 - 3 x)), {x, 0, 30}], x] (* _Vincenzo Librandi_, Aug 21 2014 *)
%o A174132 (Magma) [2*3^(n-1)-(-1)^n: n in [1..30]]; // _Vincenzo Librandi_, Aug 21 2014
%Y A174132 Cf. A154992.
%K A174132 nonn,easy
%O A174132 1,1
%A A174132 _Vincenzo Librandi_, Jan 27 2011