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 A153973 #20 Sep 08 2022 08:45:40
%S A153973 9,12,18,30,54,102,198,390,774,1542,3078,6150,12294,24582,49158,98310,
%T A153973 196614,393222,786438,1572870,3145734,6291462,12582918,25165830,
%U A153973 50331654,100663302,201326598,402653190,805306374,1610612742,3221225478
%N A153973 a(n) = 3*a(n-1) - 2*a(n-2), with a(1) = 9, a(2) = 12.
%H A153973 G. C. Greubel, <a href="/A153973/b153973.txt">Table of n, a(n) for n = 1..1000</a>
%H A153973 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2).
%F A153973 a(n) = 3*a(n-1) - 2*a(n-2), with a(1) = 9, a(2) = 12. - _Harvey P. Dale_, May 09 2012
%F A153973 From _G. C. Greubel_, Sep 01 2016: (Start)
%F A153973 a(n) = (3/2)*(4 + 2^n).
%F A153973 G.f.: 3*x*(3 - 5*x)/((1 - x)*(1 - 2*x)).
%F A153973 E.g.f.: (3/2)*(-5 + 4*exp(x) + exp(2*x)). (End)
%t A153973 a=9;lst={a};Do[a=(a-2)*2-2;AppendTo[lst,a],{n,6!}];lst
%t A153973 NestList[2#-6&,9,30] (* or *) LinearRecurrence[{3,-2},{9,12},31]
%t A153973 Table[ (3/2)*(4 + 2^n), {n, 1, 25}] (* _G. C. Greubel_, Sep 01 2016 *)
%o A153973 (Magma) I:=[9,12]; [n le 2 select I[n] else 3*Self(n-1)-2*Self(n-2): n in [1..40]]; // _Vincenzo Librandi_, Sep 01 2016
%Y A153973 Cf. A146529, A153972
%K A153973 nonn
%O A153973 1,1
%A A153973 _Vladimir Joseph Stephan Orlovsky_, Jan 03 2009
%E A153973 Definition adapted to offset by _Georg Fischer_, Jun 18 2021