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 A084428 #13 Apr 05 2019 09:33:39
%S A084428 1,23,209,1171,6239,7543
%N A084428 Numbers n such that n 7's followed by n is prime.
%C A084428 a(7) > 15000. - _Derek Orr_, Jul 29 2014
%C A084428 a(7) > 10^5. - _Robert Price_, Apr 05 2019
%e A084428 23 is in the sequence because 7777777777777777777777723(23 7's followed by 23) is prime.
%t A084428 Do[ If[GCD[n, 30] == 1 && PrimeQ[ FromDigits[ Join[ Table[7, {n}], IntegerDigits[n]]]], Print[n]], {n, 1, 6500}]
%o A084428 (PARI)
%o A084428 for(n=1,15000,p="";for(k=1,n,p=concat(p,Str(7)));p=concat(p,Str(n));if(ispseudoprime(eval(p)),print1(n,", "))) \\ _Derek Orr_, Jul 29 2014
%Y A084428 Cf. A070746, A068817.
%K A084428 nonn,base,more,hard
%O A084428 1,2
%A A084428 _Farideh Firoozbakht_, Jun 27 2003
%E A084428 a(6) from _Derek Orr_, Jul 29 2014