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 A125817 #21 Sep 08 2022 08:45:28
%S A125817 0,3,6,63,228,1527,6930,39819,197448,1071819,5500254,29221431,
%T A125817 151947180,800658687,4184419434,21980036547,115095203472,603851028243,
%U A125817 3164320515510,16594108511151,86981665785972,456063176261511,2390814670884546,12534703338214779
%N A125817 a(n) = ((1 + 3*sqrt(2))^n - (1 - 3*sqrt(2))^n)/(2*sqrt(2)).
%H A125817 G. C. Greubel, <a href="/A125817/b125817.txt">Table of n, a(n) for n = 0..1000</a> (terms 0..200 from T. D. Noe)
%H A125817 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2, 17).
%F A125817 From _Philippe Deléham_, Dec 12 2006: (Start)
%F A125817 a(n) = 2*a(n-1) + 17*a(n-2), with a(0)=0, a(1)=3.
%F A125817 G.f.: 3*x/(1-2*x-17*x^2). (End)
%t A125817 Expand[Table[((1+3Sqrt[2])^n -(1-3Sqrt[2])^n)/(2Sqrt[2]), {n, 0, 30}]]
%t A125817 LinearRecurrence[{2, 17}, {0, 3}, 30] (* _T. D. Noe_, Mar 28 2012 *)
%o A125817 (PARI) my(x='x+O('x^30)); concat([0], Vec(3*x/(1-2*x-17*x^2))) \\ _G. C. Greubel_, Aug 02 2019
%o A125817 (Magma) I:=[0,3]; [n le 2 select I[n] else 2*Self(n-1) +17*Self(n-2): n in [1..30]]; // _G. C. Greubel_, Aug 02 2019
%o A125817 (Sage) (3*x/(1-2*x-17*x^2)).series(x, 30).coefficients(x, sparse=False) # _G. C. Greubel_, Aug 02 2019
%o A125817 (GAP) a:=[0,3];; for n in [3..30] do a[n]:=2*a[n-1]+17*a[n-2]; od; a; # _G. C. Greubel_, Aug 02 2019
%K A125817 nonn
%O A125817 0,2
%A A125817 _Artur Jasinski_, Dec 10 2006