A095714 Numbers k such that 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 5, 7, 33, 45, 105, 197, 199, 281, 301, 317, 1107, 1657, 3395, 35925, 37597, 64305, 80139, 221631

Offset: 1

Author

Alonso del Arte, Jul 07 2004

Keywords

• nonn

Comments

Also numbers k such that 10^k - 9 is a prime.

Examples

			a(2) = 5, since 10^5 - 9 = 99991, which is prime.
		

Links

• Makoto Kamada, Prime numbers of the form 99...991.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093177, A056696.

Programs

Mathematica

Do[ If[ PrimeQ[10^n - 9], Print[n]], {n, 0, 7000}]

Formula

a(n) = A056696(n) + 1.

Extensions

a(12) - a(14) from Robert G. Wilson v, Oct 15 2004

a(15) - a(16) from Jason Earls, Jan 07 2008

a(17) - a(19) from Alexander Gramolin, May 13 2011

Edited by Ray Chandler, Feb 26 2012

Title corrected by Robert Price, Sep 06 2014