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

Offset

1,1

Comments

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

Terms from Kamada.

a(19) > 30000.

Links

Table of n, a(n) for n=1..18.

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

Status

approved