A256725 Numbers k such that R_k + 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 4, 7, 60, 394, 552, 1164, 1494, 5398, 7899, 11254, 13224, 77637, 118324, 120574, 142425, 142699, 157792, 188164

Offset

1,2

Comments

Also, numbers k such that (10^k + 359)/9 is prime.

Terms from Kamada data. Note that Kamada does not recognize k=1 as 41 is a degenerate case of form AAA..ABA.

a(21) > 10^6.

Links

Table of n, a(n) for n=1..20.

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

Makoto Kamada, Prime numbers of the form 11...1151.

Index entries for primes involving repunits.

Example

For k=3, R_3 + 40 = 111 + 40 = 151 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 359)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+359) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A041379 A242859 A134471 * A041463 A041863 A042543

Adjacent sequences: A256722 A256723 A256724 * A256726 A256727 A256728

Keyword

nonn,more,hard

Author

Robert Price, Apr 09 2015

Status

approved