A099415 Numbers k such that 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

12, 13, 609

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn
• bref
• more

Comments

Also numbers k such that (5*10^k - 41)/9 is prime.

a(4) > 10^5. - Robert Price, Nov 16 2014

Links

• Makoto Kamada, Prime numbers of the form 55...551.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056684.

Programs

Magma

[n: n in [0..500] | IsPrime((5*10^n-41) div 9)]; // Vincenzo Librandi, Nov 17 2014

Maple

A099415:=n->`if`(isprime((5*10^n-41)/9), n, NULL): seq(A099415(n), n=1..10^3); # Wesley Ivan Hurt, Nov 16 2014

Mathematica

Do[ If[ PrimeQ[ 5(10^n - 1)/9 - 4], Print[n]], {n, 15000}]
Select[Range[10000], PrimeQ[(5 10^# - 41) / 9] &] (* Vincenzo Librandi, Nov 17 2014 *)

Formula

a(n) = A056684(n) + 1. - Robert Price, Nov 17 2014