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 A133453 #21 Feb 03 2025 18:53:25
%S A133453 2,4,13,40,122,365,1094,3280,9841,29524,88574,265721,797162,2391484,
%T A133453 7174453,21523360,64570082,193710245,581130734,1743392200,5230176601,
%U A133453 15690529804,47071589414,141214768241,423644304722,1270932914164
%N A133453 a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 2,4,13,40.
%H A133453 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,-1,3).
%F A133453 O.g.f: -(2 - 2*x + x^2 + 3*x^3)/((3*x-1)*(x+1)*(x^2-x+1)). - _R. J. Mathar_, Nov 30 2007
%F A133453 6*a(n) = 3^(n+2) +(-1)^n +2*A057079(n+2). - _R. J. Mathar_, Oct 03 2021
%t A133453 LinearRecurrence[{3,0,-1,3},{2,4,13,40},30] (* _Harvey P. Dale_, Sep 10 2018 *)
%Y A133453 Cf. A129339.
%K A133453 nonn,easy
%O A133453 0,1
%A A133453 _Paul Curtz_, Nov 27 2007