A256800 Numbers k such that 3*R_k + 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 6, 7, 13, 22, 28, 32, 126, 172, 186, 267, 650, 693, 1083, 3783, 12294, 18134, 53859, 66650, 72097, 98890, 125706, 200001

Offset

1,2

Comments

Also, numbers k such that (10^k + 149)/3 is prime.

Terms from Kamada data. Note that Kamada does not recognize k=1 as 53 is a degenerate case of form AAA..ABA.

a(26) > 2.5*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Prime numbers of the form 33...3383.

Index entries for primes involving repunits.

Example

For k=3, 3*R_3 + 50 = 333 + 50 = 383 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 149)/3] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((10^n+149) div 3)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A233423 A328024 A309815 * A172105 A092482 A335099

Adjacent sequences: A256797 A256798 A256799 * A256801 A256802 A256803

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Status

approved