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 A087531 #14 Sep 20 2024 23:54:56
%S A087531 37,73,33373777,33773737,37373773,37377337,73337377,73337773,73373737,
%T A087531 73773373,77337373,77733373,3337737737,3337777337,3377733773,
%U A087531 3733377377,3733733777,3737733773,3773333777,3773337377,3773733737,3777333773,3777337337,3777337373
%N A087531 Primes consisting only of digits 3 and 7 occurring with equal frequency.
%C A087531 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).
%H A087531 Andrew Howroyd, <a href="/A087531/b087531.txt">Table of n, a(n) for n = 1..9185</a>
%t A087531 Select[Flatten[Table[FromDigits/@Permutations[Join[Table[3, {i}], Table[7, {i}]]], {i, 7}]], PrimeQ] (* from _Harvey P. Dale_ *)
%o A087531 (PARI) \\ Needs B() from A087510.
%o A087531 concat(vector(6,k,B(k,3,7,isprime))) \\ _Andrew Howroyd_, Sep 20 2024
%Y A087531 Cf. A087510, A087511, A087530.
%K A087531 base,nonn
%O A087531 1,1
%A A087531 _Amarnath Murthy_ and _Paul D. Hanna_, Sep 12 2003
%E A087531 Edited by _Charles R Greathouse IV_, Oct 28 2009
%E A087531 Offset changed and a(23) onwards from _Andrew Howroyd_, Sep 20 2024