A256829 Numbers k such that 7*R_k - 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 5, 8, 13, 17, 50, 169, 193, 407, 1496, 1718, 3610, 6527, 6889, 12665, 15949, 23498, 32969, 44773, 110027

Offset

1,1

Comments

Also, numbers k such that (7*10^k - 277)/9 is prime.

Terms from Kamada.

a(21) > 200000. - Tyler Busby, Apr 20 2024

Links

Table of n, a(n) for n=1..20.

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

Makoto Kamada, Prime numbers of the form 77...7747.

Index entries for primes involving repunits.

Example

For k=5, 7*R_5 - 30 = 77777 - 30 = 77747 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(7*10^# - 277)/9] &]

Prog

(Magma) [n: n in [2..500] | IsPrime((7*10^n-277) div 9)]; // Vincenzo Librandi, Apr 11 2015

Crossrefs

Cf. A002275.

Sequence in context: A225255 A076145 A332774 * A342431 A049617 A054074

Adjacent sequences: A256826 A256827 A256828 * A256830 A256831 A256832

Keyword

more,hard,nonn

Author

Robert Price, Apr 10 2015

Extensions

a(19)-a(20) from Kamada data by Tyler Busby, Apr 20 2024

Status

approved