A256721 Numbers k such that R_k + 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 6, 10, 34, 36, 370, 8256, 13290, 14724, 427912, 685224

Offset

1,2

Comments

Also, numbers k such that (10^k + 89)/9 is prime.

Terms from Kamada data. Note Kamada does not recognize k=1 as 11 is a degenerate case of form AAA..ABA.

a(12) > 10^6.

Links

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

Makoto Kamada, Near-repdigit numbers of the form AA...AABA.

Makoto Kamada, Prime numbers of the form 11...1121.

Index entries for primes involving repunits.

Example

For k=6, R_6 + 10 = 111111 + 10 = 111121 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 89)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+89) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A266957 A137272 A121801 * A192774 A218860 A242866

Adjacent sequences: A256718 A256719 A256720 * A256722 A256723 A256724

Keyword

more,hard,nonn

Author

Robert Price, Apr 09 2015

Status

approved