A056684 Numbers k such that 50*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

11, 12, 608

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn
• bref
• more

Comments

Also numbers k such that (5*10^(k+1)-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, A099415.

Programs

Mathematica

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 1], Print[n]], {n, 15000}]

Formula

a(n) = A099415(n) - 1. - Robert Price, Nov 16 2014