A259135 Numbers k such that 2*R_k + 7*10^k + 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

2, 4, 8, 28, 64, 724, 1786, 7276, 19462, 24214, 51778

Offset

1,1

Comments

Also, numbers k such that (65*10^k + 43)/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 72...27.

Index entries for primes involving repunits.

Example

For k=2, 2*R_2 + 7*10^k + 5 = 22 + 700 + 5 = 727 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(65*10^#+43)/9] &]

Crossrefs

Cf. A002275.

Sequence in context: A324328 A056800 A302915 * A219969 A034919 A298682

Adjacent sequences: A259132 A259133 A259134 * A259136 A259137 A259138

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved