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 A173802 #17 Sep 09 2024 19:47:01
%S A173802 3,53,553,5553,55553,555553,5555553,55555553,555555553,5555555553,
%T A173802 55555555553,555555555553,5555555555553,55555555555553,
%U A173802 555555555555553,5555555555555553,55555555555555553,555555555555555553,5555555555555555553,55555555555555555553,555555555555555555553
%N A173802 a(n) = (5*10^n - 23)/9.
%H A173802 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A173802 a(n) = 10*a(n-1) + 23 for n > 1.
%F A173802 G.f.: x*(3+20*x)/((10*x-1)*(x-1)). - R. J. Mathar, Aug 24 2011
%F A173802 From _Elmo R. Oliveira_, Sep 09 2024: (Start)
%F A173802 E.g.f.: exp(x)*(5*exp(9*x) - 23)/9.
%F A173802 a(n) = 11*a(n-1) - 10*a(n-2) for n > 2. (End)
%e A173802 For n=2, a(2) = 10*3 + 23 = 53.
%e A173802 For n=3, a(3) = 10*53 + 23 = 553.
%e A173802 For n=4, a(4) = 10*553 + 23 = 5553.
%t A173802 Rest[CoefficientList[Series[x*(3 + 20*x)/((10*x - 1)*(x - 1)), {x,0,50}], x]] (* _G. C. Greubel_, May 06 2017 *)
%t A173802 Table[FromDigits[PadLeft[{3},n,5]],{n,20}] (* or *) LinearRecurrence[ {11,-10},{3,53},20] (* _Harvey P. Dale_, May 27 2018 *)
%o A173802 (PARI) x=x+O('x^50); Vec(x*(3+20*x)/((10*x-1)*(x-1))) \\ _G. C. Greubel_, May 06 2017
%Y A173802 Cf. A093164.
%K A173802 nonn,easy
%O A173802 1,1
%A A173802 _Vincenzo Librandi_, Feb 25 2010