A256762 Numbers k such that R_k + 80 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 6, 27, 30, 108, 119, 209, 444, 1050, 15189, 16401, 81363, 134385, 135011

Offset

1,1

Comments

Also, numbers k such that (10^k + 719)/9 is prime.

Terms from Kamada data.

a(15) > 2.5*10^5.

Links

Table of n, a(n) for n=1..14.

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

Makoto Kamada, Prime numbers of the form 11...1191.

Index entries for primes involving repunits.

Example

For k=3, R_3 + 80 = 111 + 80 = 191 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 719)/9] &]

Prog

(Magma) [n: n in [2..300] | IsPrime((10^n+719) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A128437 A200654 A208665 * A064283 A266857 A014561

Adjacent sequences: A256759 A256760 A256761 * A256763 A256764 A256765

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Status

approved