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 A055559 #25 Mar 16 2025 13:11:56
%S A055559 2,29,2999,2999999,29999999,29999999999999999999,
%T A055559 2999999999999999999999999999,
%U A055559 29999999999999999999999999999999999999999999,29999999999999999999999999999999999999999999999999999999
%N A055559 Primes of the form 2999...999.
%C A055559 a(9)=29999999999999999999999999999999999999999999999999999999. - _Vincenzo Librandi_, Aug 07 2010
%C A055559 The next term (a(10)) has 208 digits, and a(11) has 1312 digits. - _Harvey P. Dale_, Jan 22 2023
%F A055559 a(n) = A198698(A056703(n)) = 3*10^A056703(n) - 1. - _Amiram Eldar_, Mar 16 2025
%e A055559 3*10^k - 1 is prime for k = 0, 1, 3, 6, 7, 19, ... (A056703). k gives the number of 9's in these numbers.
%t A055559 Select[Table[FromDigits[PadRight[{2}, n, 9]], {n, 60}], PrimeQ] (* _Harvey P. Dale_, Jan 22 2023 *)
%Y A055559 Cf. A051200, A056703, A056703.
%K A055559 base,nonn
%O A055559 1,1
%A A055559 _Labos Elemer_, Jul 10 2000
%E A055559 Erroneous Formula entry removed by _Jon E. Schoenfield_, Jan 14 2018
%E A055559 Extended by _Harvey P. Dale_, Jan 22 2023