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 A198699 #18 Jun 14 2025 17:15:51
%S A198699 6,69,699,6999,69999,699999,6999999,69999999,699999999,6999999999,
%T A198699 69999999999,699999999999,6999999999999,69999999999999,
%U A198699 699999999999999,6999999999999999,69999999999999999,699999999999999999,6999999999999999999,69999999999999999999,699999999999999999999
%N A198699 a(n) = 7*10^n - 1.
%H A198699 Vincenzo Librandi, <a href="/A198699/b198699.txt">Table of n, a(n) for n = 0..200</a>
%H A198699 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A198699 a(n) = 10*a(n-1) + 9.
%F A198699 a(n) = 11*a(n-1) - 10*a(n-2), n>1.
%F A198699 G.f.: (6+3*x)/((10*x-1)*(x-1)). - _R. J. Mathar_, Oct 29 2011
%F A198699 From _Elmo R. Oliveira_, Jun 14 2025: (Start)
%F A198699 E.g.f.: exp(x)*(7*exp(9*x) - 1).
%F A198699 a(n) = 3*A198972(n). (End)
%t A198699 NestList[10#+9&,6,20] (* or *) LinearRecurrence[{11,-10},{6,69},20] (* _Harvey P. Dale_, Dec 29 2013 *)
%o A198699 (Magma) [7*10^n-1: n in [0..30]];
%Y A198699 Cf. A002283, A109002, A198972.
%K A198699 nonn,easy
%O A198699 0,1
%A A198699 _Vincenzo Librandi_, Oct 29 2011