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 A199689 #20 May 04 2025 14:44:37
%S A199689 9,81,801,8001,80001,800001,8000001,80000001,800000001,8000000001,
%T A199689 80000000001,800000000001,8000000000001,80000000000001,
%U A199689 800000000000001,8000000000000001,80000000000000001,800000000000000001,8000000000000000001,80000000000000000001,800000000000000000001
%N A199689 a(n) = 8*10^n + 1.
%H A199689 Vincenzo Librandi, <a href="/A199689/b199689.txt">Table of n, a(n) for n = 0..1000</a>
%H A199689 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A199689 a(n) = 9*A059482(n).
%F A199689 a(n) = 10*a(n-1) - 9.
%F A199689 a(n) = 11*a(n-1) - 10*a(n-2).
%F A199689 G.f.: 9*(1-2*x)/((1-x)*(1-10*x)).
%F A199689 From _Elmo R. Oliveira_, May 04 2025: (Start)
%F A199689 E.g.f.: exp(x)*(1 + 8*exp(9*x)).
%F A199689 a(n) = 3*A199688(n). (End)
%t A199689 8*10^Range[0,20]+1 (* or *) LinearRecurrence[{11,-10},{9,81},20] (* _Harvey P. Dale_, Jun 11 2022 *)
%o A199689 (Magma) [8*10^n+1: n in [0..30]];
%Y A199689 Cf. A059482, A199688.
%K A199689 nonn,easy
%O A199689 0,1
%A A199689 _Vincenzo Librandi_, Nov 09 2011