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 A143967 #7 Aug 28 2017 15:16:16
%S A143967 3,7,33,37,73,77,333,337,373,377,733,737,773,777,3333,3337,3373,3377,
%T A143967 3733,3737,3773,3777,7333,7337,7373,7377,7733,7737,7773,7777,33333,
%U A143967 33337,33373,33377,33733,33737,33773,33777,37333,37337,37373,37377
%N A143967 Numbers containing only digits 3 or 7 in decimal representation.
%C A143967 See A020463 for primes.
%H A143967 Reinhard Zumkeller, <a href="/A143967/b143967.txt">Table of n, a(n) for n = 1..10000</a>
%F A143967 a(n) = f(n+1, 0) with f(n, x) = if n=1 then A004086(x) else f(floor(n/2), 10*x + 3 + 4*(n mod 2)).
%t A143967 Table[FromDigits/@Tuples[{3,7},n],{n,5}]//Flatten (* _Harvey P. Dale_, Aug 28 2017 *)
%o A143967 (Haskell)
%o A143967 a143967 = f 0 . (+ 1) where
%o A143967 f y 1 = a004086 y
%o A143967 f y x = f (10 * y + 3 + 4 * r) x' where (x', r) = divMod x 2
%o A143967 -- _Reinhard Zumkeller_, Mar 18 2015
%Y A143967 Cf. A004086, A020463, A032810.
%K A143967 base,nonn
%O A143967 1,1
%A A143967 _Reinhard Zumkeller_, Sep 06 2008