A056683 Numbers k such that 40*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 4, 5, 47, 107, 244, 1043, 20207, 52739, 89188

Offset

1,1

Comments

Also numbers k such that (4*10^(k+1) + 41)/9 is prime.

a(11) > 10^5. - Robert Price, Nov 09 2014

Links

Table of n, a(n) for n=1..10.

Makoto Kamada, Prime numbers of the form 44...449.

Index entries for primes involving repunits

Formula

a(n) = A099414(n+1) - 1. - Robert Price, Nov 09 2014

Mathematica

Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

Prog

(Magma) [n: n in [0..300] | IsPrime((4*10^(n+1)+41)div 9)]; Vincenzo Librandi, Nov 10 2014

Crossrefs

Cf. A002275, A099414.

Sequence in context: A154775 A210418 A167454 * A065574 A247963 A105288

Adjacent sequences: A056680 A056681 A056682 * A056684 A056685 A056686

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

a(8)-a(10) derived from A099414 by Robert Price, Nov 09 2014

Status

approved