A256761 Numbers k such that R_k + 70 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 4, 7, 9, 10, 16, 124, 243, 303, 459, 471, 520, 591, 1726, 2971, 5560, 5757, 13222

Offset

1,2

Comments

Also, numbers k such that (10^k + 629)/9 is prime.

Terms from Kamada data. Note that Kamada does not recognize k=1 as 81 is a degenerate case of form AAA..ABA.

a(20) > 2.5*10^5.

Links

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Prime numbers of the form 11...1181.

Index entries for primes involving repunits.

Example

For k=3, R_3 + 70 = 111 + 70 = 181 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 629)/9] &]

Prog

(Magma) [n: n in [1..350] | IsPrime((10^n+629) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A024799 A240531 A212013 * A039579 A274794 A115104

Adjacent sequences: A256758 A256759 A256760 * A256762 A256763 A256764

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Status

approved