%I
%S 1,23,209,1171,6239,7543
%N Numbers n such that n 7's followed by n is prime.
%C a(7) > 15000.  _Derek Orr_, Jul 29 2014
%C a(7) > 10^5.  _Robert Price_, Apr 05 2019
%e 23 is in the sequence because 7777777777777777777777723(23 7's followed by 23) is prime.
%t Do[ If[GCD[n, 30] == 1 && PrimeQ[ FromDigits[ Join[ Table[7, {n}], IntegerDigits[n]]]], Print[n]], {n, 1, 6500}]
%o (PARI)
%o 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 Cf. A070746, A068817.
%K nonn,base,more,hard
%O 1,2
%A _Farideh Firoozbakht_, Jun 27 2003
%E a(6) from _Derek Orr_, Jul 29 2014
