A256426 Numbers k such that 7*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 3, 5, 17, 44, 47, 78, 504, 533, 768, 915, 1155, 1313, 7128, 8088, 15717, 33074, 43568, 58689, 82517, 85647, 154211

Offset

1,1

Comments

Also, numbers k such that (7*10^k + 173)/9 is prime.

Terms from Kamada.

a(23) > 200000. - Tyler Busby, Apr 20 2024

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Prime numbers of the form 77...7797.

Index entries for primes involving repunits.

Example

For k=3, 7*R_11 + 20 = 777 + 20 = 797 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(7*10^# + 173)/9] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((7*10^n + 173) div 9)]; // Vincenzo Librandi, Apr 13 2015

Crossrefs

Cf. A002275.

Sequence in context: A339855 A076706 A243441 * A019350 A235630 A235636

Adjacent sequences: A256423 A256424 A256425 * A256427 A256428 A256429

Keyword

more,hard,nonn

Author

Robert Price, Apr 12 2015

Extensions

a(18)-a(22) from Kamada data by Tyler Busby, Apr 20 2024

Status

approved