A259119 Numbers k such that 8*R_k + 10^k - 7 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 2, 8, 14, 40, 92, 128, 884, 9424, 14768, 19258, 31234

Offset

1,2

Comments

Also, numbers k such that (17*10^k - 71)/9 is prime.

Terms from Kamada data.

a(13) > 10^5.

Links

Table of n, a(n) for n=1..12.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 188...881.

Index entries for primes involving repunits.

Example

For k=8, 8*R_8 + 10^k - 7 = 88888888 + 100000000 - 7 = 188888881 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(17*10^#-71)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A332611 A162484 A333575 * A039660 A333053 A333054

Adjacent sequences: A259116 A259117 A259118 * A259120 A259121 A259122

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved