A056687 Numbers k such that 50*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 7, 11, 17, 25, 31, 137, 187, 221, 337, 1001, 2743, 6529, 38689, 39463

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• nonn

Comments

Also numbers k such that (5*10^(k+1)+31)/9 is prime.

Links

• Makoto Kamada, Prime numbers of the form 55...559.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093403, A099418.

Programs

Mathematica

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]

Formula

a(n) = A099418(n) - 1.

Extensions

6529 from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

38689 and 39463 from Serge Batalov, Jan 06 2009 confirmed as next terms by Ray Chandler, Feb 20 2012