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 A168535 #10 Jun 22 2019 15:02:49
%S A168535 2,1212121,12121212121,1212121212121212121212121212121212121212121
%N A168535 Primes in A028359.
%C A168535 Except for the first term, each term must both begin and end with a "1" because each term greater than 2 must end in a "1" and if a term begins with a "2" the sum of the digits of the term will be divisible by 3 and thus the term is not a prime. - _Harvey P. Dale_, Jun 22 2019
%C A168535 The next term (a(5)) has 139 digits and a(6) has 627 digits. - _Harvey P. Dale_, Jun 22 2019
%t A168535 Join[{2},Select[Table[FromDigits[PadRight[{},2n+1,{1,2}]],{n,30}],PrimeQ]] (* _Harvey P. Dale_, Jun 22 2019 *)
%Y A168535 Cf. A028359.
%K A168535 nonn
%O A168535 1,1
%A A168535 _Zak Seidov_, Nov 28 2009