A097685 Numbers k such that R_k + 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 5, 6, 8, 17, 50, 684, 720, 1452, 1679, 3146, 72821, 111902, 146063, 180689, 330906

Offset

1,1

Comments

Also numbers k such that (10^k + 71)/9 is prime.

The corresponding values R_k + 8 are primes of the form "(k-1) ones followed by a nine". Related to the base-10 repunit primes.

a(13) > 10^5. - Robert Price, Nov 01 2014

Links

Table of n, a(n) for n=1..16.

Makoto Kamada, Prime numbers of the form 11...119.

Index entries for primes involving repunits

Formula

a(n) = A056659(n) + 1.

Mathematica

Do[ If[ PrimeQ[(10^n - 1)/9 + 8], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 14 2004 *)

Crossrefs

Cf. A004023, A056659, A097683, A097684.

Sequence in context: A191140 A271430 A326854 * A136369 A305511 A269472

Adjacent sequences: A097682 A097683 A097684 * A097686 A097687 A097688

Keyword

nonn,more,hard

Author

Carl R. White, Aug 19 2004

Extensions

a(12) from Robert Price, Nov 01 2014

a(13)-a(16) from Kamada data by Tyler Busby, May 03 2024

Status

approved