A259140 Numbers k such that 2*R_k + 9*10^k + 7 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

2, 6, 12, 110, 3608, 37784

Offset

1,1

Comments

Also, numbers k such that (83*10^k + 61)/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 922...229.

Index entries for primes involving repunits.

Example

For k=2, 2*R_2 + 9*10^k + 7 = 22 + 900 + 7 = 929 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(83*10^#+61)/9] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((83*10^n+61) div 9)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A141288 A038787 A144098 * A075071 A250269 A193987

Adjacent sequences: A259137 A259138 A259139 * A259141 A259142 A259143

Keyword

nonn,more,hard

Author

Robert Price, Jun 18 2015

Status

approved