A099416 Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 8, 26, 66, 74, 233, 473, 540, 2774, 64715, 102492, 141594

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn

Comments

Also numbers k such that (5*10^k-23)/9 is prime.

a(12) > 10^5. - Robert Price, Nov 13 2014

Links

• Makoto Kamada, Prime numbers of the form 55...553.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056685.

Programs

Mathematica

Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]

Formula

a(n) = A056685(n) + 1.

Extensions

a(11) from Robert Price, Nov 13 2014

0 removed by Georg Fischer, Jan 03 2021

a(12)-a(13) from Kamada data by Tyler Busby, May 03 2024