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 A306855 #13 May 15 2021 15:29:01
%S A306855 19,109,199,1009,1999,10009,10099,100999,199999,1000099,1000999,
%T A306855 19999999,1000000009,1000009999,1000099999,1009999999,10000000999,
%U A306855 10000099999,10999999999,100999999999,1000000009999,1000000999999,1099999999999,10000000000099,10009999999999,100000000000099,100000009999999
%N A306855 Primes of the form 10^i + 10^j - 1.
%C A306855 Primes p such that p+1 is in A052216.
%C A306855 Primes of the following form in base 10: 1, followed by 0 or more 0's, then 1 or more 9's.
%H A306855 Robert Israel, <a href="/A306855/b306855.txt">Table of n, a(n) for n = 1..2000</a>
%p A306855 select(isprime, [seq(seq(10^n+10^m-1, m=1..n),n=1..15)]);
%t A306855 Select[Flatten[Table[FromDigits[Join[PadRight[{1},n,0],PadRight[{},m,9]]],{n,20},{m,20}]],PrimeQ]//Sort (* _Harvey P. Dale_, May 15 2021 *)
%Y A306855 Cf. A052216. Subsequence of A199329.
%K A306855 nonn,base
%O A306855 1,1
%A A306855 _Robert Israel_, Mar 13 2019