A257041 Numbers k such that 9*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 4, 5, 9, 19, 25, 34, 70, 82, 718, 2035, 2494, 3369, 3953, 4219, 8524, 30943, 49375, 52082, 139670, 303255

Offset

1,1

Comments

Also, numbers k such that 99*10^k - 1 is prime.

Terms from Kamada.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Prime numbers of the form 9899...99.

Index entries for primes involving repunits.

Example

For k=3, 9*R_5 - 10^3 = 99999 - 1000 = 98999 which is prime.

Mathematica

Select[Range[0, 280000], PrimeQ[99*10^#-1 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime(99*10^n-1)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A080552 A215176 A063781 * A346603 A056797 A029727

Adjacent sequences: A257038 A257039 A257040 * A257042 A257043 A257044

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Extensions

a(21) from Predrag Kurtovic, May 24 2015

Status

approved