A056659 Numbers k such that 10*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 4, 5, 7, 16, 49, 683, 719, 1451, 1678, 3145, 72820

Offset: 1

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (10^(k+1)+71)/9 is prime.

a(13) > 10^5. - Robert Price, Nov 01 2014

Links

• Makoto Kamada, Prime numbers of the form 11...119.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A097685.

Programs

Mathematica

Do[ If[ PrimeQ[10*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

Formula

a(n) = A097685(n) - 1. - Robert Price, Nov 01 2014

Extensions

a(12) derived from A097685 by Robert Price, Nov 01 2014