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

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

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

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

Links

• Makoto Kamada, Prime numbers of the form 33...337.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093168, A099411.

Programs

Mathematica

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

Formula

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

Extensions

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