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A084428 Numbers n such that n 7's followed by n is prime. 7

%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

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Last modified June 18 13:28 EDT 2021. Contains 345112 sequences. (Running on oeis4.)