A256828 Numbers k such that 7*R_k - 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 6, 18, 48, 204, 10386, 19524, 28320

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 367)/9 is prime.

Terms from Kamada.

a(9) > 30000.

a(9) > 2*10^5. - Tyler Busby, Feb 02 2023

Links

Table of n, a(n) for n=1..8.

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

Makoto Kamada, Prime numbers of the form 77...7737.

Index entries for primes involving repunits.

Example

For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &]

Prog

(Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A218759 A295499 A059413 * A197055 A258625 A062026

Adjacent sequences: A256825 A256826 A256827 * A256829 A256830 A256831

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Status

approved