A257031 Numbers k such that 7*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 5, 10, 11, 16, 23, 247, 1700, 2891, 3019, 5549, 5837, 9326, 14417, 23312, 24155, 30740, 61907, 64421, 69997, 163106, 177266

Offset

1,2

Comments

Also, numbers k such that (682*10^k - 7)/9 is prime.

Terms from Kamada.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Prime numbers of the form 7577...77.

Index entries for primes involving repunits.

Example

For k=5, 7*R_7 - 2*10^5 = 7777777 - 200000 = 7577777 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(682*10^#-7)/9 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((682*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A163624 A353356 A363890 * A167799 A179871 A258116

Adjacent sequences: A257028 A257029 A257030 * A257032 A257033 A257034

Keyword

more,hard,nonn,changed

Author

Robert Price, Apr 14 2015

Extensions

a(18)-a(23) from Kamada data by Tyler Busby, Apr 16 2024

Status

approved