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 A036949 #18 Mar 09 2020 12:27:22
%S A036949 7,0,787,7877,78787,777787,7778777,77778887,777788887,7777778887,
%T A036949 77777778887,777777777877,7777777787777,77777777778787,
%U A036949 777777777777787,7777777778788777,77777777777778887,777777777787878887
%N A036949 Smallest n-digit prime containing only the digits 7 and 8, or 0 if no such prime exists.
%H A036949 Jinyuan Wang, <a href="/A036949/b036949.txt">Table of n, a(n) for n = 1..500</a>
%t A036949 Table[SelectFirst[FromDigits/@Tuples[{7,8},n],PrimeQ],{n,18}]/.Missing[ "NotFound"]->0 (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Oct 07 2018 *)
%Y A036949 Cf. A020470, A036229, A036323.
%K A036949 nonn,base
%O A036949 1,1
%A A036949 _Patrick De Geest_, Jan 04 1999