A259126 Numbers k such that 5*R_k + 3*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

2, 8, 140, 230, 426, 462, 726, 1974, 7230, 45860, 47304

Offset

1,1

Comments

Also, numbers k such that (32*10^k - 23)/9 is prime.

Terms from Kamada data.

a(12) > 10^5.

Links

Table of n, a(n) for n=1..11.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 355...553.

Index entries for primes involving repunits.

Example

For k=2, 5*R_2 + 3*10^k - 2 = 55 + 300 - 2 = 353 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(32*10^#-23)/9] &]

Prog

(Magma) [n: n in [0..450] | IsPrime((32*10^n-23) div 9)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A111827 A045330 A193203 * A140050 A318038 A304798

Adjacent sequences: A259123 A259124 A259125 * A259127 A259128 A259129

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved