A256830 Numbers k such that 7*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 4, 24, 106, 124, 162, 243, 258, 1344, 1386, 2494, 4200, 5859, 8844, 13122, 19908, 86844, 106066, 180732

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 187)/9 is prime.

Terms from Kamada.

a(17) > 30000.

a(20) > 2*10^5. - Tyler Busby, Feb 01 2023

Links

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Prime numbers of the form 77...7757.

Index entries for primes involving repunits.

Example

For k=4, 7*R_4 - 20 = 7777 - 20 = 7757 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(7*10^# - 187)/9] &]

Prog

(Magma) [n: n in [2..400] | IsPrime((7*10^n-187) div 9)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A124632 A048091 A189738 * A065900 A065809 A093600

Adjacent sequences: A256827 A256828 A256829 * A256831 A256832 A256833

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Extensions

a(17)-a(19) from Tyler Busby, Feb 01 2023

Status

approved