A256724 Numbers k such that R_k + 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 8, 56, 675470, 718580

Offset

1,2

Comments

Also, numbers k such that (10^k + 269)/9 is prime.

Terms from Kamada data. Note that Kamada does not recognize k=1 as 31 is a degenerate case of form AAA..ABA.

a(7) > 10^6.

Links

Table of n, a(n) for n=1..6.

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

Makoto Kamada, Prime numbers of the form 11...1141.

Index entries for primes involving repunits.

Example

For k=8, R_8 + 30 = 11111111 + 30 = 11111141 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 269)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+269) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A191508 A113725 A345677 * A153538 A153566 A153529

Adjacent sequences: A256721 A256722 A256723 * A256725 A256726 A256727

Keyword

more,hard,nonn

Author

Robert Price, Apr 09 2015

Status

approved