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 A214488 #14 Jul 09 2018 21:22:20
%S A214488 13,31,1133,1313,1331,3113,3131,3311,111333,113133,113313,113331,
%T A214488 131133,131313,131331,133113,133131,133311,311133,311313,311331,
%U A214488 313113,313131,313311,331113,331131,331311,333111
%N A214488 List of words over {1,3} with equal numbers of 1's and 3's.
%C A214488 This is to 3 as A214218 is to 2. The subsequence of primes (when interpreted as decimal integers) is A087511.
%H A214488 Robert Israel, <a href="/A214488/b214488.txt">Table of n, a(n) for n = 1..10000</a>
%p A214488 sort([seq(seq((10^(2*d)-1)/9+2*add(10^i, i=s), s=combinat:-choose([$0..(2*d-1)], d)), d=1..4)]); # _Robert Israel_, Jul 09 2018
%t A214488 Flatten[Table[Sort[FromDigits/@Permutations[PadRight[{},2n,{1,3}]]], {n,3}]] (* _Harvey P. Dale_, Aug 03 2014 *)
%Y A214488 Cf. A214218.
%K A214488 nonn,base,easy
%O A214488 1,1
%A A214488 _Jonathan Vos Post_, Jul 18 2012