A257028 Numbers k such that 7*R_(k+2) - 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 4, 10, 28, 49, 64, 79, 109, 169, 1270, 5638, 6812, 7951, 11737, 16360, 22840, 25394, 33394

Offset

1,2

Comments

Also, numbers k such that (655*10^k - 7)/9 is prime.

Terms from Kamada.

a(18) > 30000.

Links

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

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

Makoto Kamada, Prime numbers of the form 7277...77.

Index entries for primes involving repunits.

Example

For k=4, 7*R_6 - 5*10^2 = 777777 - 50000 = 727777 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(655*10^#-7)/9 ] &]

Prog

(PARI) for(n=0, 200, if(isprime((655*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..400] | IsPrime((655*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A367124 A222963 A333172 * A174938 A032248 A092504

Adjacent sequences: A257025 A257026 A257027 * A257029 A257030 A257031

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Extensions

a(18) from Kamada data by Tyler Busby, Apr 17 2024

Status

approved