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 A087511 #22 Sep 21 2024 12:39:42
%S A087511 13,31,11313331,11333131,13111333,13131133,13131331,13133311,13311313,
%T A087511 31133131,33113131,1113131333,1131131333,1131311333,1131331133,
%U A087511 1133111333,1133113133,1133133311,1133311313,1133313113,1133313131,1133331113,1311113333,1311311333,1311313313
%N A087511 Primes consisting only of digits 1 and 3 occurring with equal frequency.
%C A087511 There are 18 digit pairs which can produce such primes. (1, 0), (1, 3), (1, 4), (1, 6), (1, 7), (1, 9), (2, 3), (2, 9), (3, 4), (3, 5), (3, 7), (3, 8), (4, 7), (4, 9), (5, 9), (6, 7), (7, 9), (8, 9). - corrected by _Robert Israel_, Jul 10 2018
%C A087511 The number of digits is even and not divisible by 3. - _Robert Israel_, Jul 09 2018
%H A087511 Robert Israel, <a href="/A087511/b087511.txt">Table of n, a(n) for n = 1..10000</a>
%p A087511 sort(select(isprime,[seq(seq((10^(2*d)-1)/9+2*add(10^i, i=s), s=combinat:-choose([$0..(2*d-1)], d)), d=[1,2,4,5,7,8,10])])); # _Robert Israel_, Jul 09 2018
%t A087511 Union[FromDigits/@Select[Flatten[Table[Tuples[{1,3},k],{k,10}],1], PrimeQ[FromDigits[#]] && Count[#,1]==Count[#,3] &]] (* _Jayanta Basu_, May 19 2013 *)
%o A087511 (PARI) \\ Needs B() from A087510.
%o A087511 concat(vector(6,k,B(k,1,3,isprime))) \\ _Andrew Howroyd_, Sep 21 2024
%Y A087511 Cf. A087510, A087512, A087513.
%K A087511 base,nonn
%O A087511 1,1
%A A087511 _Paul D. Hanna_ and _Amarnath Murthy_, Sep 11 2003