A259130 Numbers k such that 4*R_k + 3*10^k - 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

0, 6, 12, 492, 5568, 24756

Offset

1,2

Comments

Also, numbers k such that (31*10^k - 13)/9 is prime.

Terms from Kamada data.

Note that Kamada does not recognize k=0 as 2 is a degenerate case of form ABB..BBA.

a(7) > 2*10^5.

Links

Table of n, a(n) for n=1..6.

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

Makoto Kamada, Prime numbers of the form 344...443.

Index entries for primes involving repunits.

Example

For k=6, 4*R_6 + 3*10^k - 1 = 444444 + 3000000 - 1 = 3444443 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(31*10^#-13)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A334809 A325030 A191415 * A032511 A036900 A159282

Adjacent sequences: A259127 A259128 A259129 * A259131 A259132 A259133

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved