A256760 Numbers k such that R_k + 60 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 4, 5, 8, 17, 19, 64, 368, 616, 857, 4351, 6526, 10555, 12685, 14549, 33886

Offset

1,2

Comments

Also, numbers k such that (10^k + 539)/9 is prime.

Terms from Kamada data. Note that Kamada does not recognize k=1 as 61 is a degenerate case of form AAA..ABA.

a(18) > 2.5*10^5.

Links

Table of n, a(n) for n=1..17.

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

Makoto Kamada, Prime numbers of the form 11...1171.

Index entries for primes involving repunits.

Example

For k=5, R_5 + 60 = 11111 + 60 = 11171 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 539)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+539) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A045581 A253426 A092061 * A281646 A240460 A239405

Adjacent sequences: A256757 A256758 A256759 * A256761 A256762 A256763

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Status

approved