A257032 Numbers k such that 7*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 9, 132, 8547, 13215, 14110

Offset

1,1

Comments

Also, numbers k such that (691*10^k-7)/9 is prime.

Terms from Kamada.

a(7) > 30000.

Links

Table of n, a(n) for n=1..6.

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

Makoto Kamada, Prime numbers of the form 7677...77.

Index entries for primes involving repunits.

Example

For k=3, 7*R_5 - 10^3 = 77777 - 1000 = 76777 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(691*10^#-7)/9 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((691*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A082606 A062702 A289169 * A141143 A368092 A163402

Adjacent sequences: A257029 A257030 A257031 * A257033 A257034 A257035

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved