A096506 Numbers n for which 2*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 2, 3, 8, 11, 36, 95, 101, 128, 260, 351, 467, 645, 1011, 1178, 1217, 2442, 3761, 3806, 15617, 26459, 63117, 88545, 93497

Offset: 1

Author

Labos Elemer, Jul 12 2004

Keywords

• nonn

Comments

Also numbers n such that (2*10^n + 7)/9 is prime.

Per Kamada link, 181457, 202059, 262874 are also terms, found by Rytis Slatkevicius. - Michael S. Branicky, Sep 13 2024

Examples

			n=36: 222222222222222222222222222222222223 is a prime number.
		

Links

• Makoto Kamada, Prime numbers of the form 22...223.
• Index entries for primes involving repunits

Crossrefs

Cf. A056656, A093162, A096503, A096504, A096505, A096507, A096508.

Programs

Mathematica

Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 1], Print[n]], {n, 7000}] (* Robert G. Wilson v, Oct 14 2004 *)

Formula

a(n) = A056656(n) + 1.

Extensions

a(20)-a(24) from Kamada link by Ray Chandler, Feb 27 2012