A259123 Numbers k such that 4*R_k + 10^k - 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

1, 6, 66, 1254, 8406, 67038

Offset

1,2

Comments

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

Terms from Kamada data.

a(7) > 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 144...441.

Index entries for primes involving repunits.

Example

For k=6, 4*R_6 + 10^k - 3 = 444444 + 1000000 - 3 = 1444441 which is prime.

Mathematica

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

Crossrefs

Cf. A002275.

Sequence in context: A329930 A367308 A271220 * A292026 A126459 A054969

Adjacent sequences: A259120 A259121 A259122 * A259124 A259125 A259126

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved