A056659 Numbers k such that 10*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 4, 5, 7, 16, 49, 683, 719, 1451, 1678, 3145, 72820, 111901, 146062, 180688, 330905

Offset

1,2

Comments

Also numbers k such that (10^(k+1)+71)/9 is prime.

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) = A097685(n) - 1. - Robert Price, Nov 01 2014

Mathematica

Do[ If[ PrimeQ[10*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

Crossrefs

Cf. A002275, A097685.

Sequence in context: A216537 A085195 A107289 * A292321 A049896 A029520

Adjacent sequences: A056656 A056657 A056658 * A056660 A056661 A056662

Keyword

nonn,hard,more

Author

Robert G. Wilson v, Aug 09 2000

Extensions

a(12) derived from A097685 by Robert Price, Nov 01 2014

a(13) from Lyle Blosser, Dec 06 2025

a(14)-a(16) from A097685 by Michael S. Branicky, Dec 06 2025

Status

approved