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 A133448 #22 Feb 03 2025 18:53:38
%S A133448 2,5,13,40,121,365,1094,3281,9841,29524,88573,265721,797162,2391485,
%T A133448 7174453,21523360,64570081,193710245,581130734,1743392201,5230176601,
%U A133448 15690529804,47071589413,141214768241,423644304722,1270932914165,3812798742493,11438396227480
%N A133448 a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 2,5,13,40.
%H A133448 Robert Israel, <a href="/A133448/b133448.txt">Table of n, a(n) for n = 0..2093</a>
%H A133448 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,-1,3).
%F A133448 G.f.: ( -2+x+2*x^2-3*x^3 ) / ( (3*x-1)*(1+x)*(x^2-x+1) ). - _Robert Israel_, Jun 08 2017
%F A133448 6*a(n) = -(-1)^n +3^(n+2) +2*A057079(n+1). - _R. J. Mathar_, Oct 03 2021
%p A133448 f:= gfun:-rectoproc({a(n)=3*a(n-1)-a(n-3)+3*a(n-4),a(0)=2,a(1)=5,a(2)=13,a(3)=40,a(4)=121},a(n),remember):
%p A133448 map(f, [$0..40]); # _Robert Israel_, Jun 08 2017
%K A133448 nonn,easy
%O A133448 0,1
%A A133448 _Paul Curtz_, Nov 27 2007