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A115099 a(0)=4, a(n) = 3*a(n-1) - 4.

%I A115099 #23 Jun 26 2023 19:32:32
%S A115099 4,8,20,56,164,488,1460,4376,13124,39368,118100,354296,1062884,
%T A115099 3188648,9565940,28697816,86093444,258280328,774840980,2324522936,
%U A115099 6973568804,20920706408,62762119220,188286357656,564859072964,1694577218888,5083731656660,15251194969976
%N A115099 a(0)=4, a(n) = 3*a(n-1) - 4.
%C A115099 A tetrahedron has 4 faces. Cut every corner so that we get triangular faces; the resulting polyhedron has 8 faces. Repeating this procedure gives polyhedra with 4, 8, 20, 56, etc. faces.
%H A115099 Vincenzo Librandi, <a href="/A115099/b115099.txt">Table of n, a(n) for n = 0..300</a>
%H A115099 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (4,-3).
%F A115099 a(n) = 2*3^n + 2.
%F A115099 From _Colin Barker_, May 31 2016: (Start)
%F A115099 a(n) = 4*a(n-1)-3*a(n-2) for n>1.
%F A115099 G.f.: 4*(1-2*x) / ((1-x)*(1-3*x)).
%F A115099 (End)
%F A115099 E.g.f.: 2*(1 + exp(2*x))*exp(x). - _Ilya Gutkovskiy_, May 31 2016
%F A115099 a(n) = 4 * A007051(n). - _Alois P. Heinz_, Jun 26 2023
%p A115099 seq(2*3^i+2,i=0..30);
%t A115099 a=4;lst={a};Do[a=a*3-4;AppendTo[lst,a],{n,0,5!}];lst (* _Vladimir Joseph Stephan Orlovsky_, Dec 25 2008 *)
%o A115099 (Magma) [2*3^n+2: n in [0..30]]; // _Vincenzo Librandi_, Jun 05 2011
%o A115099 (PARI) Vec(4*(1-2*x)/((1-x)*(1-3*x)) + O(x^30)) \\ _Colin Barker_, May 31 2016
%Y A115099 Cf. A003462, A007051, A034472, A024023, A067771, A029858, A134931. - _Vladimir Joseph Stephan Orlovsky_, Dec 25 2008
%K A115099 easy,nonn
%O A115099 0,1
%A A115099 _Miklos Kristof_, Mar 02 2006