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 A165553 #12 Apr 22 2024 09:41:43
%S A165553 1,3,-3,15,-39,123,-363,1095,-3279,9843,-29523,88575,-265719,797163,
%T A165553 -2391483,7174455,-21523359,64570083,-193710243,581130735,-1743392199,
%U A165553 5230176603,-15690529803,47071589415,-141214768239
%N A165553 a(n) = (3/2)*(1+(-3)^(n-1)).
%C A165553 a(n)/a(n-1) tends to -3.
%H A165553 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (-2,3).
%F A165553 a(0)=1, a(1)=3, a(n)=3*a(n-2)-2*a(n-1).
%F A165553 G.f.: (1+5x)/(1+2x-3x^2).
%F A165553 a(n)= Sum_{k=0..n} A112555(n,k)*2^(n-k).
%t A165553 3/2*(1 + (-3)^(Range[0, 29] - 1)) (* or *)
%t A165553 LinearRecurrence[{-2, 3}, {1, 3}, 30] (* _Paolo Xausa_, Apr 22 2024 *)
%Y A165553 Cf. A046717, A084247, A112555.
%K A165553 easy,sign
%O A165553 0,2
%A A165553 _Philippe Deléham_, Sep 21 2009