A096508 Numbers k for which 8*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 14, 17, 35, 4175, 4472, 9812, 12260, 12341, 13760, 14576, 53411, 144683, 148328

Offset: 1

Author

Labos Elemer, Jul 12 2004

Keywords

• nonn

Comments

Also numbers k such that (8*10^k + 1)/9 is prime.

a(15) > 2*10^5. - Robert Price, Sep 06 2014

Examples

			35 is a term because 88888888888888888888888888888888889 (34 8's) is a prime number.
		

Links

• Makoto Kamada, Prime numbers of the form 88...889.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056663, A096503-A096507.

Programs

Maple

select(n -> isprime((8*10^n+1)/9), [$1..10000]); # Robert Israel, Sep 07 2014

Mathematica

Do[ If[ PrimeQ[ 8(10^n - 1)/9 + 1], Print[n]], {n, 0, 30000}] (* Robert G. Wilson v, Oct 15 2004 *)

PARI

for(n=1,10^4,if(ispseudoprime(8*(10^n-1)/9+1),print1(n,", "))) \\ Derek Orr, Sep 06 2014

Formula

a(n) = A056663(n) + 1.

Extensions

Four missing terms (9812, 12260, 12341, 13760) added, and a(12)-a(14) added from Kamada data, by Robert Price, Sep 06 2014