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

1,2

Comments

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

Links

Table of n, a(n) for n=1..15.

Makoto Kamada, Prime numbers of the form 55...559.

Index entries for primes involving repunits

Formula

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

Mathematica

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

Crossrefs

Cf. A002275, A093403, A099418.

Sequence in context: A088176 A046132 A162337 * A088981 A257666 A224868

Adjacent sequences: A056684 A056685 A056686 * A056688 A056689 A056690

Keyword

nonn

Author

Robert G. Wilson v, Aug 10 2000

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

Status

approved