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 A015561 #34 Dec 30 2023 23:41:32
%S A015561 0,1,7,53,399,3005,22631,170437,1283583,9666829,72802135,548282261,
%T A015561 4129184367,31097419613,234198674759,1763780401765,13283257511391,
%U A015561 100037924186797,753398499353143,5673941192219189,42731182342946895
%N A015561 Expansion of x/(1 - 7*x - 4*x^2).
%H A015561 Vincenzo Librandi, <a href="/A015561/b015561.txt">Table of n, a(n) for n = 0..1000</a>
%H A015561 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (7,4).
%F A015561 a(n) = 7*a(n-1) + 4*a(n-2).
%t A015561 Join[{a=0,b=1},Table[c=7*b+4*a;a=b;b=c,{n,100}]] (* _Vladimir Joseph Stephan Orlovsky_, Jan 17 2011 *)
%t A015561 LinearRecurrence[{7, 4}, {0, 1}, 30] (* _Vincenzo Librandi_, Nov 14 2012 *)
%o A015561 (Sage) [lucas_number1(n,7,-4) for n in range(0, 21)] # _Zerinvary Lajos_, Apr 24 2009
%o A015561 (Magma) [n le 2 select n-1 else 7*Self(n-1) + 4*Self(n-2): n in [1..30]]; // _Vincenzo Librandi_, Nov 14 2012
%o A015561 (PARI) x='x+O('x^30); concat([0], Vec(x/(1-7*x-4*x^2))) \\ _G. C. Greubel_, Dec 30 2017
%K A015561 nonn,easy
%O A015561 0,3
%A A015561 _Olivier Gérard_