A256711 Numbers k such that R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 8, 12, 17, 86, 146, 1428, 1949, 4809, 16922, 33102, 125792, 211610

Offset

1,1

Comments

Also, numbers k such that (10^k - 91)/9 is prime.

Terms from Kamada data.

a(14) > 2.5*10^5.

Links

Table of n, a(n) for n=1..13.

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

Makoto Kamada, Prime numbers of the form 11...1101.

Index entries for primes involving repunits.

Example

For k=3, R_3 - 10 = 111 - 10 = 101 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# - 91)/9] &]

Prog

(Magma) [n: n in [0..1400] | IsPrime((10^n-91) div 9)]; // Vincenzo Librandi, Apr 09 2015

Crossrefs

Cf. A002275.

Sequence in context: A190374 A189758 A190368 * A219635 A063225 A213238

Adjacent sequences: A256708 A256709 A256710 * A256712 A256713 A256714

Keyword

more,hard,nonn

Author

Robert Price, Apr 08 2015

Status

approved