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 A098576 #16 Sep 08 2022 08:45:15
%S A098576 1,1,1,1,4,10,19,31,55,109,220,424,793,1489,2845,5473,10480,19954,
%T A098576 37963,72391,138259,263989,503608,960400,1831969,3495505,6669865,
%U A098576 12725425,24276892,46314874,88362451,168586303,321640831,613639981,1170726484
%N A098576 a(n) = Sum_{k=0..floor(n/4)} C(n-2*k,2*k) * 3^k.
%H A098576 G. C. Greubel, <a href="/A098576/b098576.txt">Table of n, a(n) for n = 0..1000</a>
%H A098576 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,0,3).
%F A098576 G.f.: (1-x)/((1-x)^2-3*x^4).
%F A098576 a(n) = 2*a(n-1) - a(n-2) + 3*a(n-4).
%t A098576 Table[Sum[Binomial[n-2k,2k]3^k,{k,0,Floor[n/4]}],{n,0,40}] (* or *) LinearRecurrence[{2,-1,0,3},{1,1,1,1},40] (* _Harvey P. Dale_, Dec 04 2011 *)
%o A098576 (PARI) x='x+O('x^30); Vec((1-x)/((1-x)^2-3*x^4)) \\ _G. C. Greubel_, Feb 03 2018
%o A098576 (Magma) I:=[1,1,1,1]; [n le 4 select I[n] else 2*Self(n-1) - Self(n-2) + 3*Self(n-4): n in [1..30]]; // _G. C. Greubel_, Feb 03 2018
%Y A098576 Cf. A097116, A098575.
%K A098576 easy,nonn
%O A098576 0,5
%A A098576 _Paul Barry_, Sep 16 2004