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 A133594 #13 Jan 02 2024 08:55:16
%S A133594 1,3,18,90,486,2538,13446,70794,373734,1970730,10396998,54839754,
%T A133594 289283238,1525926762,8049179142,42458658570,223966125414,
%U A133594 1181402279082,6231800342214,32872228375626,173398289233446,914661608207850
%N A133594 a(n)=3*a(n-1)+12*a(n-2) for n>=3, a(0)=1, a(1)=3, a(2)=18 .
%H A133594 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (3,12).
%F A133594 G.f.: (1-3*x^2)/(1-3*x-12*x^2) . a(n) = Sum_{k, 0<=k<=n}A122950(n,k)*3^k .
%t A133594 LinearRecurrence[{3,12},{1,3,18},30] (* _Harvey P. Dale_, Dec 20 2020 *)
%K A133594 easy,nonn
%O A133594 0,2
%A A133594 _Philippe Deléham_, Dec 31 2007