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 A173770 #14 Jun 28 2025 01:00:10
%S A173770 3,43,443,4443,44443,444443,4444443,44444443,444444443,4444444443,
%T A173770 44444444443,444444444443,4444444444443,44444444444443,
%U A173770 444444444444443,4444444444444443,44444444444444443,444444444444444443,4444444444444444443,44444444444444444443,444444444444444444443
%N A173770 a(n) = (4*10^n - 13)/9.
%H A173770 Paolo Xausa, <a href="/A173770/b173770.txt">Table of n, a(n) for n = 1..990</a>
%H A173770 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A173770 a(n) = 10*a(n-1) + 13 (with a(1)=3).
%F A173770 G.f.: x*(3+10*x)/((10*x-1)*(x-1)). - _R. J. Mathar_, Aug 23 2011
%F A173770 From _Elmo R. Oliveira_, Jun 19 2025: (Start)
%F A173770 E.g.f.: 1 + exp(x)*(4*exp(9*x) - 13)/9.
%F A173770 a(n) = 11*a(n-1) - 10*a(n-2) for n > 2. (End)
%e A173770 For n=2, a(2)=10*3+13=43; n=3, a(3)=10*43+13=443; n=4, a(4)=10*443+13=4443.
%t A173770 (4*10^Range[25] - 13)/9 (* _Paolo Xausa_, Jun 27 2025 *)
%o A173770 (PARI) my(x='x+O('x^22)); Vec(x*(3+10*x)/((1-11*x+10*x^2))) \\ _Elmo R. Oliveira_, Jun 19 2025
%Y A173770 Cf. A093163.
%K A173770 nonn,easy
%O A173770 1,1
%A A173770 _Vincenzo Librandi_, Feb 24 2010
%E A173770 More terms from _Elmo R. Oliveira_, Jun 19 2025