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

Offset

1,2

Comments

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

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

Links

Table of n, a(n) for n=1..11.

Makoto Kamada, Prime numbers of the form 55...553.

Index entries for primes involving repunits

Formula

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

Mathematica

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

Crossrefs

Cf. A002275, A056685.

Sequence in context: A216929 A100504 A327600 * A211885 A101696 A155861

Adjacent sequences: A099413 A099414 A099415 * A099417 A099418 A099419

Keyword

nonn

Author

Robert G. Wilson v, Oct 14 2004

Extensions

a(11) from Robert Price, Nov 13 2014

0 removed by Georg Fischer, Jan 03 2021

Status

approved