A056726 Numbers k such that 9*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 5, 20, 41, 47, 92, 161, 401, 455, 8570, 18592, 73714, 78635, 100780

Offset

1,1

Comments

Also numbers k such that (82*10^k - 1)/9 is prime.

Links

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 911...11.

Index entries for primes involving repunits

Mathematica

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

Crossrefs

Cf. A002275, A093634.

Sequence in context: A192164 A244087 A361134 * A176042 A185593 A136899

Adjacent sequences: A056723 A056724 A056725 * A056727 A056728 A056729

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

8570 from Hugo Pfoertner, Oct 22 2004

a(11)-a(14) from Robert Price, Dec 05 2014

Status

approved