A259139 Numbers k such that 8*R_k + 9*10^k + 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

6, 72, 96, 114, 204, 984, 1226, 4794, 20720, 133580, 411590

Offset

1,1

Comments

Also, numbers k such that (89*10^k + 1)/9 is prime.

Terms from Kamada data.

a(12) > 7*10^5.

Links

Table of n, a(n) for n=1..11.

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

Makoto Kamada, Prime numbers of the form 988...889.

Index entries for primes involving repunits.

Example

For k=6, 8*R_6 + 9*10^k - 2 = 888888 + 9000000 + 1 = 9888889 which is prime.

Mathematica

Select[Range[0, 700000], PrimeQ[(89*10^#+1)/9] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((89*10^n+1)div 9)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A024272 A167813 A242232 * A052615 A340506 A338535

Adjacent sequences: A259136 A259137 A259138 * A259140 A259141 A259142

Keyword

more,hard,nonn

Author

Robert Price, Jun 18 2015

Status

approved