A056680 Numbers k such that 30*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 5, 45, 393, 977, 2585, 2810, 2967, 3641, 4826, 4917, 5591, 5705, 10682, 12890, 14117, 74349, 88679

Offset

1,3

Comments

Also numbers k such that (10^(k+1)+11)/3 is prime.

a(21) > 10^5. - Robert Price, Nov 02 2014

Links

Table of n, a(n) for n=1..20.

Makoto Kamada, Prime numbers of the form 33...337.

Index entries for primes involving repunits

Formula

a(n) = A099411(n) - 1. - Robert Price, Nov 02 2014

Mathematica

Do[ If[ PrimeQ[30*(10^n - 1)/9 + 7], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A093168, A099411.

Sequence in context: A088309 A334252 A307147 * A005166 A121621 A225147

Adjacent sequences: A056677 A056678 A056679 * A056681 A056682 A056683

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

a(16)-a(20) derived from A099411 by Robert Price, Nov 02 2014

Status

approved