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 A098589 #11 Sep 08 2022 08:45:15
%S A098589 1,3,9,29,93,297,949,3033,9693,30977,98997,316377,1011085,3231249,
%T A098589 10326501,33001673,105467517,337055553,1077170005,3442445049,
%U A098589 11001446253,35158678769,112360926405,359085671721,1147574372701
%N A098589 a(n) = 3*a(n-1) + 2*a(n-3), with a(0)=1, a(1)=3.
%H A098589 G. C. Greubel, <a href="/A098589/b098589.txt">Table of n, a(n) for n = 0..1000</a>
%H A098589 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,2).
%F A098589 G.f.: 1/(1-3*x-2*x^3).
%F A098589 a(n) = Sum_{k=0..floor(n/2)} Sum_{i=0..n} Sum_{j=0..i} C(n-2*k,i)*C(i,j) *C(j, k)*2^k.
%t A098589 CoefficientList[Series[1/(1-3*x-2*x^3), {x,0,50}], x] (* or *) LinearRecurrence[{3,0,2},{1,3,9}, 50] (* _G. C. Greubel_, Feb 03 2018 *)
%o A098589 (PARI) x='x+O('x^30); Vec(1/(1-3*x-2*x^3)) \\ _G. C. Greubel_, Feb 03 2018
%o A098589 (Magma) I:=[1,3,9]; [n le 3 select I[n] else 3*Self(n-1) + 2*Self(n-3): n in [1..30]]; // _G. C. Greubel_, Feb 03 2018
%Y A098589 Cf. A052541.
%K A098589 easy,nonn
%O A098589 0,2
%A A098589 _Paul Barry_, Sep 16 2004