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

1,2

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

Links

Table of n, a(n) for n=1..24.

Makoto Kamada, Prime numbers of the form 22...223.

Index entries for primes involving repunits

Formula

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

Example

n=36: 222222222222222222222222222222222223 is a prime number.

Mathematica

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

Crossrefs

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

Sequence in context: A041123 A042045 A317912 * A119155 A119195 A119199

Adjacent sequences: A096503 A096504 A096505 * A096507 A096508 A096509

Keyword

nonn

Author

Labos Elemer, Jul 12 2004

Extensions

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

Status

approved