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 A093400 #18 Jul 08 2021 06:24:04
%S A093400 19,11119,111119,11111119,11111111111111119,
%T A093400 11111111111111111111111111111111111111111111111119
%N A093400 Primes of the form 10*R_k + 9, where R_k is the repunit (A002275) of length k.
%C A093400 The next term has 684 digits. - _Harvey P. Dale_, Dec 11 2013
%H A093400 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.
%H A093400 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A093400 Select[Table[FromDigits[PadLeft[{9},n,1]],{n,1000}],PrimeQ] (* _Harvey P. Dale_, Dec 11 2013 *)
%Y A093400 Cf. A056659 (corresponding k and count of digits 1 in a(n)).
%K A093400 nonn
%O A093400 1,1
%A A093400 _Rick L. Shepherd_, Mar 28 2004