A070746 Numbers k such that k 1's followed by k is a prime.

1, 7, 709, 2203, 4481, 5107, 11489

Offset: 1

Author

Robert G. Wilson v, May 03 2002

Keywords

• base
• nonn
• more

Comments

The number corresponding to a(3) has been certified prime with Primo; those corresponding to a(4), a(5), a(6) are probable primes.

a(8) > 147000, if it exists. - Giovanni Resta, Jun 22 2018

Examples

			7 is in the sequence because 11111117 (seven 1's followed by 7) is prime.
		

Crossrefs

Cf. A068817, A084428.

Programs

Mathematica

Do[ If[ PrimeQ[ FromDigits[ Join[ IntegerDigits[(10^n - 1)/9], IntegerDigits[n]]]], Print[n]], {n, 1, 2000, 2}]

PARI

isok(n) = isprime(eval(concat(apply(x->Str(x), vector(n+1, j, if (j>n, n, 1)))))) \\ Michel Marcus, Jun 15 2018

WinPFGW

Primality testing r(5107)*10^len(5107)+5107 [N-1/N+1, Brillhart-Lehmer-Selfridge] r(5107)*10^len(5107)+5107 is Fermat and Lucas PRP! (79 seconds). - Jason Earls

Extensions

a(4) from Jason Earls, May 06 2002

a(5) from Rick L. Shepherd, Sep 02 2002

a(6) from Farideh Firoozbakht, Jun 29 2003

a(7) from Farideh Firoozbakht, Jul 04 2003

Edited by T. D. Noe, Oct 30 2008