A256801 Numbers k such that 7*R_k - 60 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 4, 11, 23, 86, 148, 191, 232, 271, 656, 844, 1069, 1318, 1348, 1411, 2329, 4120, 4831, 12691, 14695, 17719, 39614, 139417

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 547)/9 is prime.

Terms from Kamada.

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Prime numbers of the form 77...7717.

Makoto Kamada, Search for 7w17.

Index entries for primes involving repunits.

Example

For k=4, 7*R_4 - 60 = 7777 - 60 = 7717, which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(7*10^# - 547)/9] &]

Prog

(Magma) [n: n in [2..300] | IsPrime((7*10^n-547) div 9)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A298255 A298926 A292157 * A103669 A230711 A034485

Adjacent sequences: A256798 A256799 A256800 * A256802 A256803 A256804

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Extensions

a(22) from Robert Price, May 16 2017

a(23) from Robert Price, Jan 26 2018

Status

approved