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A256801 Numbers k such that 7*R_k - 60 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
2, 4, 11, 23, 86, 148, 191, 232, 271, 656, 844, 1069, 1318, 1348, 1411, 2329, 4120, 4831, 12691, 14695, 17719, 39614, 139417 (list; graph; refs; listen; history; text; internal format)
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

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Last modified July 31 13:25 EDT 2021. Contains 346373 sequences. (Running on oeis4.)