A256726 Numbers k such that 7*R_k - 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 4, 7, 12, 15, 48, 87, 138, 342, 675, 901, 1323, 1362, 1485, 1909, 9498, 10941, 11563, 16308, 37495, 43875, 71166, 82960

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 457)/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...7727.

Index entries for primes involving repunits.

Example

For k=4, 7*R_4 - 50 = 7777 - 50 = 7727 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(7*10^# - 457)/9] &]

Prog

(Magma) [n: n in [2..400] | IsPrime((7*10^n - 457) div 9)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A192112 A034885 A280991 * A130324 A020677 A362222

Adjacent sequences: A256723 A256724 A256725 * A256727 A256728 A256729

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Extensions

a(21)-a(23) from Robert Price, Sep 28 2017

Status

approved