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 A173804 #26 Sep 09 2024 12:28:52
%S A173804 1,51,551,5551,55551,555551,5555551,55555551,555555551,5555555551,
%T A173804 55555555551,555555555551,5555555555551,55555555555551,
%U A173804 555555555555551,5555555555555551,55555555555555551,555555555555555551,5555555555555555551,55555555555555555551,555555555555555555551
%N A173804 a(n) = (5*10^n - 41)/9 for n > 0.
%C A173804 Primes up to n=100 are 555555555551 and 5555555555551; the next prime term has 609 digits (see A056684).
%H A173804 Vincenzo Librandi, <a href="/A173804/b173804.txt">Table of n, a(n) for n = 1..100</a>
%H A173804 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A173804 a(n) = 10*a(n-1) + 41 with a(0)=-4.
%F A173804 From _Vincenzo Librandi_, Jul 05 2012: (Start)
%F A173804 G.f.: x*(1+40*x)/((1-x)*(1-10*x)).
%F A173804 a(n) = 11*a(n-1) - 10*a(n-2) for n > 2. (End)
%F A173804 E.g.f.: 4 + exp(x)*(5*exp(9*x) - 41)/9. - _Elmo R. Oliveira_, Sep 09 2024
%t A173804 CoefficientList[Series[(1+40*x)/((1-x)*(1-10*x)),{x,0,30}],x] (* _Vincenzo Librandi_, Jul 05 2012 *)
%t A173804 Table[FromDigits[PadLeft[{1},n,5]],{n,20}] (* or *) LinearRecurrence[ {11,-10},{1,51},20](* _Harvey P. Dale_, Dec 04 2021 *)
%o A173804 (Magma)[(5*10^n-41)/9: n in [1..20]]; // _Vincenzo Librandi_, Jul 05 2012
%Y A173804 Cf. A056684 (numbers n such that (5*10^(n+1)-41)/9 is prime). - _Klaus Brockhaus_, Feb 28 2010
%K A173804 nonn,easy
%O A173804 1,2
%A A173804 _Vincenzo Librandi_, Feb 25 2010