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 A002447 #23 Sep 08 2022 08:44:30
%S A002447 1,0,2,3,4,12,17,36,70,123,248,456,865,1656,3098,5907,11164,21108,
%T A002447 40049,75708,143422,271563,513968,973392,1842625,3488688,6605426,
%U A002447 12505251,23676916,44826780,84869585,160684308
%N A002447 Expansion of 1/(1-2*x^2-3*x^3).
%H A002447 Vincenzo Librandi, <a href="/A002447/b002447.txt">Table of n, a(n) for n = 0..1000</a>
%H A002447 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,3).
%F A002447 From _Paul Barry_, Oct 16 2004: (Start)
%F A002447 a(n) = 2*a(n-2) + 3*a(n-3).
%F A002447 a(n) = Sum_{k=0..floor(n/2)} binomial(k, n-2*k)*2^k*(3/2)^(n-2*k). (End)
%t A002447 CoefficientList[Series[1/(1-2*x^2-3*x^3),{x,0,40}],x] (* _Vincenzo Librandi_, Jun 11 2012 *)
%t A002447 LinearRecurrence[{0,2,3}, {1,0,2}, 40] (* _G. C. Greubel_, Jul 04 2019 *)
%o A002447 (Magma) I:=[1, 0, 2]; [n le 3 select I[n] else 2*Self(n-2)+3*Self(n-3): n in [1..40]]; // _Vincenzo Librandi_, Jun 11 2012
%o A002447 (PARI) Vec(1/(1-2*x^2-3*x^3)+O(x^40)) \\ _Charles R Greathouse IV_, Sep 26 2012
%o A002447 (Sage) (1/(1-2*x-3*x^3)).series(x, 40).coefficients(x, sparse=False) # _G. C. Greubel_, Jul 04 2019
%o A002447 (GAP) a:=[1,0,2];; for n in [4..40] do a[n]:=2*a[n-2]+3*a[n-3]; od; a; # _G. C. Greubel_, Jul 04 2019
%K A002447 nonn,easy
%O A002447 0,3
%A A002447 _N. J. A. Sloane_