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 A036944 #15 Oct 10 2019 03:57:20
%S A036944 7,47,0,4447,44777,0,4444747,44447747,0,4444444447,44444444747,0,
%T A036944 4444444447777,44444447447447,0,4444444447447777,44444444444444477,0,
%U A036944 4444444444444444777,44444444444444444447,0,4444444444444444444747,44444444444444444444477,0
%N A036944 Smallest n-digit prime containing only the digits 4 and 7, or 0 if no such prime exists.
%H A036944 Alois P. Heinz, <a href="/A036944/b036944.txt">Table of n, a(n) for n = 1..500</a>
%t A036944 Flatten[Table[Select[FromDigits/@Tuples[{4,7},n],PrimeQ,1],{n,22}]/.{}->{0}] (* _Harvey P. Dale_, Jun 28 2012 *)
%Y A036944 Cf. A036229, A020465, A036318.
%K A036944 nonn,base
%O A036944 1,1
%A A036944 _Patrick De Geest_, Jan 04 1999