A257033 Numbers k such that 7*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 4, 5, 7, 11, 25, 31, 109, 205, 518, 1288, 1697, 1711, 1775, 1997, 6784, 8741, 13171, 20275, 67517

Offset

1,2

Comments

Also, numbers k such that (709*10^k - 7)/9 is prime.

Terms from Kamada.

a(22) > 2*10^5.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Prime numbers of the form 7877...77.

Index entries for primes involving repunits.

Example

For k=4, 7*R_6 + 10^4 = 777777 + 10000 = 787777 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(709*10^#-7)/9 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((709*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A164316 A323146 A250006 * A039674 A090649 A154686

Adjacent sequences: A257030 A257031 A257032 * A257034 A257035 A257036

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Extensions

a(21) from Robert Price, Sep 28 2017

Status

approved