A256722 Numbers k such that R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 3, 5, 8, 9, 11, 18, 20, 29, 39, 123, 153, 267, 327, 627, 1020, 1686, 1764, 2960, 18236, 34613, 53991, 67404, 119880, 221654, 230706

Offset

1,1

Comments

Also, numbers k such that (10^k + 179)/9 is prime.

Terms from Kamada data.

a(27) > 2.5*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Prime numbers of the form 11...1131.

Index entries for primes involving repunits.

Example

For k=3, R_3 + 20 = 111 + 20 = 131 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 179)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+179) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A226815 A284890 A051214 * A364926 A363097 A192391

Adjacent sequences: A256719 A256720 A256721 * A256723 A256724 A256725

Keyword

more,hard,nonn

Author

Robert Price, Apr 09 2015

Status

approved