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 A173808 #20 Dec 03 2024 16:57:25
%S A173808 9,79,779,7779,77779,777779,7777779,77777779,777777779,7777777779,
%T A173808 77777777779,777777777779,7777777777779,77777777777779,
%U A173808 777777777777779,7777777777777779,77777777777777779,777777777777777779,7777777777777777779,77777777777777777779,777777777777777777779
%N A173808 a(n) = (7*10^n + 11)/9 for n > 0.
%H A173808 Vincenzo Librandi, <a href="/A173808/b173808.txt">Table of n, a(n) for n = 1..100</a>
%H A173808 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A173808 a(n) = 10*a(n-1) - 11 with n > 0, a(0)=2.
%F A173808 From _Vincenzo Librandi_, Jul 05 2012: (Start)
%F A173808 G.f.: x*(9-20*x)/((1-x)*(1-10*x)).
%F A173808 a(n) = 11*a(n-1) - 10*a(n-2) for n > 2. (End)
%F A173808 E.g.f.: exp(x)*(7*exp(9*x) + 11)/9 - 2. - _Elmo R. Oliveira_, Sep 09 2024
%t A173808 CoefficientList[Series[(9-20*x)/((1-x)*(1-10*x)),{x,0,30}],x] (* _Vincenzo Librandi_, Jul 05 2012 *)
%t A173808 LinearRecurrence[{11,-10},{9,79},30] (* or *) Table[10*FromDigits[PadRight[{},n,7]]+9,{n,0,30}] (* _Harvey P. Dale_, Dec 03 2024 *)
%o A173808 (Magma) [(7*10^n+11)/9: n in [1..20]]; // _Vincenzo Librandi_ Jul 05 2012
%Y A173808 Cf. A093404.
%K A173808 nonn,easy
%O A173808 1,1
%A A173808 _Vincenzo Librandi_, Feb 25 2010