A056689 Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 4, 8, 11, 14, 20, 263, 382, 2719, 4493, 21166, 45824, 55850, 64567, 70726

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (7*10^(k+1)-43)/9 is prime.

a(18) > 10^5. - Robert Price, Nov 01 2014

Links

• Makoto Kamada, Prime numbers of the form 77...773.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093165, A099420.

Programs

Mathematica

Do[ If[ PrimeQ[70*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

Formula

a(n) = A099420(n) - 1. - Robert Price, Nov 01 2014

Extensions

a(13)-a(17) derived from A099420 by Robert Price, Nov 01 2014