A259137 Numbers k such that 8*R_k + 7*10^k - 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

2, 4, 86, 112, 170, 566, 1688, 8902, 115810

Offset

1,1

Comments

Also, numbers k such that (71*10^k - 17)/9 is prime.

Terms from Kamada data.

Links

Table of n, a(n) for n=1..9.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 788...887.

Index entries for primes involving repunits.

Example

For k=2, 8*R_2 + 7*10^k - 1 = 88 + 700 - 1 = 787 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(71*10^#-17)/9] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((71*10^n-17)div 9)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A012435 A121103 A018397 * A009217 A015170 A013146

Adjacent sequences: A259134 A259135 A259136 * A259138 A259139 A259140

Keyword

more,hard,nonn

Author

Robert Price, Jun 18 2015

Status

approved