A256772 Numbers k such that 3*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 3, 4, 7, 9, 37, 39, 67, 73, 183, 216, 1161, 2622, 2775, 6240, 22078, 30384, 189385

Offset

1,1

Comments

Also, numbers k such that (10^k - 61)/3 is prime.

Terms from Kamada data.

a(19) > 2.5*10^5.

Links

Table of n, a(n) for n=1..18.

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

Makoto Kamada, Prime numbers of the form 33...3313.

Index entries for primes involving repunits.

Example

For k=3, 3*R_3 - 20 = 333 - 20 = 313 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# - 61)/3] &]

Prog

(Magma) [n: n in [2..300] | IsPrime((10^n-61) div 3)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A319645 A071114 A078696 * A274976 A269729 A098115

Adjacent sequences: A256769 A256770 A256771 * A256773 A256774 A256775

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Status

approved