A259133 Numbers k such that 6*R_k + 7*10^k + 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

4, 6, 54, 96, 454, 574, 3384, 11440, 12624, 19446, 35460, 81214, 95326

Offset

1,1

Comments

Also, numbers k such that (23*10^k + 1)/3 is prime.

Terms from Kamada data.

a(14) > 7*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 766...667.

Index entries for primes involving repunits.

Example

For k=4, 6*R_4 + 7*10^k + 1 = 6666 + 70000 + 1 = 76667 which is prime.

Mathematica

Select[Range[0, 700000], PrimeQ[(23*10^#+1)/3] &]

Crossrefs

Cf. A002275.

Sequence in context: A013164 A013024 A012910 * A131847 A351733 A089630

Adjacent sequences: A259130 A259131 A259132 * A259134 A259135 A259136

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved