A256773 Numbers k such that 3*R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 4, 6, 10, 12, 14, 17, 41, 69, 141, 170, 188, 204, 233, 306, 555, 743, 1908, 2006, 2140, 2772, 8105, 15170, 40038, 41908, 48604, 51652, 54038, 56781, 129051, 283342

Offset

1,1

Comments

Also, numbers k such that (10^k - 31)/3 is prime.

Terms from Kamada data.

a(32) > 3*10^5.

Links

Table of n, a(n) for n=1..31.

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

Makoto Kamada, Prime numbers of the form 33...3323.

Index entries for primes involving repunits.

Example

For k=4, 3*R_4 - 10 = 3333 - 10 = 3323 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# - 31)/3] &]

Prog

(Magma) [n: n in [2..310] | IsPrime((10^n-31) div 3)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A022305 A103799 A175817 * A141104 A047410 A339331

Adjacent sequences: A256770 A256771 A256772 * A256774 A256775 A256776

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Extensions

a(31) from Robert Price, Jul 12 2023

Status

approved