A256723 Numbers k such that R_k + 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 5, 8, 18, 38, 1919, 1934, 4250, 4397, 12626, 13151, 26693, 81353, 83916

Offset

1,1

Comments

Also, numbers k such that (10^k + 449)/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...1161.

Index entries for primes involving repunits.

Example

For k=5, R_5 + 50 = 11111 + 50 = 11161 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(10^# + 449)/9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((10^n+449) div 9)]; // Vincenzo Librandi, Apr 10 2015

Crossrefs

Cf. A002275.

Sequence in context: A152006 A271619 A197211 * A032063 A320426 A133147

Adjacent sequences: A256720 A256721 A256722 * A256724 A256725 A256726

Keyword

more,hard,nonn

Author

Robert Price, Apr 09 2015

Status

approved