A056663 Numbers k such that 80*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 13, 16, 34, 4174, 4471, 9811, 12259, 12340, 13759, 14575, 53410, 144682, 148327

Offset: 1

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (8*10^(k+1) + 1)/9 is prime.

a(15) > 2*10^5. - Robert Price, Nov 01 2014

Links

• Makoto Kamada, Prime numbers of the form 88...889.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A096508.

Programs

Mathematica

Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 9 ], Print[n]], {n, 0, 30000}]

Formula

a(n) = A096508(n) - 1. - Robert Price, Nov 01 2014

Extensions

Intermediate missing terms 9811 (from M. Kamada), 12259, 12340, 13759 added by Serge Batalov, Dec 14 2008

a(12)-a(14) derived from A096508 by Robert Price, Nov 01 2014