A257041 - OEIS

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

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OFFSET

1,1

COMMENTS

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

Terms from Kamada.

LINKS

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

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

EXTENSIONS

a(21) from Predrag Kurtovic, May 24 2015

STATUS

approved