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 A198970 #29 Sep 17 2024 15:41:21
%S A198970 3,39,399,3999,39999,399999,3999999,39999999,399999999,3999999999,
%T A198970 39999999999,399999999999,3999999999999,39999999999999,
%U A198970 399999999999999,3999999999999999,39999999999999999,399999999999999999,3999999999999999999,39999999999999999999,399999999999999999999
%N A198970 a(n) = 4*10^n - 1.
%H A198970 Vincenzo Librandi, <a href="/A198970/b198970.txt">Table of n, a(n) for n = 0..200</a>
%H A198970 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A198970 a(n) = 3*A097166(n).
%F A198970 a(n) = 10*a(n-1) + 9, n > 0.
%F A198970 a(n) = 11*a(n-1) - 10*a(n-2), n > 1.
%F A198970 G.f.: 3*(1 + 2*x)/(1 - 11*x + 10*x^2). - _Vincenzo Librandi_, Jan 03 2013
%F A198970 E.g.f.: exp(x)*(4*exp(9*x) - 1). - _Elmo R. Oliveira_, Sep 10 2024
%t A198970 CoefficientList[Series[3*(1 + 2*x)/(1 - 11*x + 10*x^2), {x, 0, 30}], x] (* _Vincenzo Librandi_, Jan 03 2013 *)
%o A198970 (Magma) [4*10^n-1 : n in [0..20]];
%Y A198970 Cf. A097166.
%K A198970 nonn,easy
%O A198970 0,1
%A A198970 _Vincenzo Librandi_, Nov 01 2011