A257040 Numbers k such that 9*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 90, 1928, 2206, 7244, 11110, 125056, 131394

Offset

1,2

Comments

Also, numbers k such that 98*10^k - 1 is prime.

Terms from Kamada.

a(9) > 300000.

Links

Table of n, a(n) for n=1..8.

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

Makoto Kamada, Prime numbers of the form 9799...99.

Index entries for primes involving repunits.

Example

For k=0, 9*R_2 - 2*10^0 = 99 - 2 = 97 which is prime.

Mathematica

Select[Range[0, 300000], PrimeQ[98*10^#-1 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime(98*10^n-1)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A201062 A240259 A065951 * A008393 A055603 A210090

Adjacent sequences: A257037 A257038 A257039 * A257041 A257042 A257043

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved