A256912 Numbers k such that R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 4, 7, 10, 16, 66, 138, 5839, 23898, 112098

Offset

1,2

Comments

Also, numbers k such that (91*10^k - 1)/9 is prime.

Terms from Kamada.

a(12) > 350000.

Links

Table of n, a(n) for n=1..11.

Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.

Makoto Kamada, Prime numbers of the form 1011...11.

Index entries for primes involving repunits.

Example

For k=3, R_5 - 10^3 = 11111 - 1000 = 10111 which is prime.

Mathematica

Select[Range[0, 350000], PrimeQ[(91*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((91*10^n - 1) div 9)]; // Vincenzo Librandi, Apr 13 2015

Crossrefs

Cf. A002275.

Sequence in context: A241654 A047967 A282893 * A134591 A058611 A357459

Adjacent sequences: A256909 A256910 A256911 * A256913 A256914 A256915

Keyword

more,hard,nonn

Author

Robert Price, Apr 12 2015

Status

approved