A259118 Numbers k such that 6*R_k + 10^k - 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 4, 12, 16, 18, 36, 52, 72, 100, 6232, 24028, 40222, 66394

Offset

1,2

Comments

Also, numbers k such that (5*10^k - 17)/3 is prime.

Terms from Kamada data.

a(14) > 2*10^5.

Links

Table of n, a(n) for n=1..13.

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

Makoto Kamada, Prime numbers of the form 166...661.

Index entries for primes involving repunits.

Example

For k=4, 6*R_4 + 10^k - 5 = 6666 + 10000 - 5 = 16661 which is prime.

Mathematica

Select[Range[0, 200000], PrimeQ[(5*10^#-17)/3] &]

Prog

(Magma) [n: n in [0..200] | IsPrime((5*10^n-17) div 3)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A193022 A145199 A082420 * A205683 A310564 A053362

Adjacent sequences: A259115 A259116 A259117 * A259119 A259120 A259121

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved