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 A081632 #14 Sep 08 2022 08:45:09
%S A081632 1,7,17,55,161,487,1457,4375,13121,39367,118097,354295,1062881,
%T A081632 3188647,9565937,28697815,86093441,258280327,774840977,2324522935,
%U A081632 6973568801,20920706407,62762119217,188286357655,564859072961,1694577218887
%N A081632 2*3^n-(-1)^n.
%C A081632 Binomial transform of A081631. Inverse binomial transform of A081654.
%H A081632 Vincenzo Librandi, <a href="/A081632/b081632.txt">Table of n, a(n) for n = 0..1000</a>
%H A081632 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,3).
%F A081632 G.f.: (1+5*x)/((1-3*x)(1+x)).
%F A081632 E.g.f. 2*exp(3*x)-exp(-x).
%F A081632 a(0)=1, a(1)=7, a(n)=2*a(n-1)+3*a(n-2) [ _Harvey P. Dale_, May 24 2011]
%t A081632 Total/@Partition[Riffle[2*3^Range[0,30],{-1,1}],2] (* or *) LinearRecurrence[{2,3},{1,7},30] (* _Harvey P. Dale_, May 24 2011 *)
%t A081632 CoefficientList[Series[(1 + 5 x) / ((1 - 3 x) (1 + x)), {x, 0, 40}], x] (* _Vincenzo Librandi_, Aug 10 2013 *)
%o A081632 (Magma) [2*3^n-(-1)^n: n in [0..40]]; // _Vincenzo Librandi_, Aug 10 2013
%K A081632 easy,nonn
%O A081632 0,2
%A A081632 _Paul Barry_, Mar 26 2003