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

1, 23, 209, 1171, 6239, 7543

Offset: 1

Author

Farideh Firoozbakht, Jun 27 2003

Keywords

• nonn
• base
• more
• hard

Comments

a(7) > 15000. - Derek Orr, Jul 29 2014

a(7) > 10^5. - Robert Price, Apr 05 2019

Examples

			23 is in the sequence because 7777777777777777777777723(23 7's followed by 23) is prime.
		

Crossrefs

Cf. A070746, A068817.

Programs

Mathematica

Do[ If[GCD[n, 30] == 1 && PrimeQ[ FromDigits[ Join[ Table[7, {n}], IntegerDigits[n]]]], Print[n]], {n, 1, 6500}]

PARI

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

Extensions

a(6) from Derek Orr, Jul 29 2014