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 A090149 #7 Jan 24 2019 15:15:33
%S A090149 11,13,17,19,199,1777,1999,199999,19999999,1777777777,17777777777777,
%T A090149 1333333333333333,1111111111111111111,11111111111111111111111,
%U A090149 199999999999999999999999999,1999999999999999999999999999
%N A090149 Primes of the form identical digits preceded by a 1.
%H A090149 Vincenzo Librandi, <a href="/A090149/b090149.txt">Table of n, a(n) for n = 1..38</a>
%t A090149 Select[FromDigits/@Flatten[Table[PadRight[{1},i,#]&/@{1,3,7,9},{i,2,40}],1],PrimeQ[#]&]
%K A090149 nonn,base
%O A090149 1,1
%A A090149 _Robert G. Wilson v_, Nov 22 2003
%E A090149 Mathematica program modified by _Harvey P. Dale_, Jan 24 2019