A259125 Numbers k such that 7*R_k + 3*10^k - 4 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

2, 14, 54, 68, 84, 86, 156, 2766, 3380, 3876, 5208, 10746, 15768, 31316, 40958, 45804, 46566, 51008, 80162

Offset

1,1

Comments

Also, numbers k such that (34*10^k - 43)/9 is prime.

Terms from Kamada data.

a(20) > 10^5.

Links

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Prime numbers of the form 377...773.

Index entries for primes involving repunits.

Example

For k=2, 7*R_2 + 3*10^k - 4 = 77 + 300 - 4 = 373 which is prime.

Mathematica

Select[Range[0, 100000], PrimeQ[(34*10^#-43)/9] &]

Prog

(Magma) [n: n in [0..450] | IsPrime((34*10^n-43) div 9)]; // Vincenzo Librandi, Jun 19 2015

Crossrefs

Cf. A002275.

Sequence in context: A341493 A064363 A363706 * A067056 A208428 A356373

Adjacent sequences: A259122 A259123 A259124 * A259126 A259127 A259128

Keyword

nonn,hard,more

Author

Robert Price, Jun 18 2015

Status

approved