A099420 Numbers k such that 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 5, 9, 12, 15, 21, 264, 383, 2720, 4494, 21167, 45825, 55851, 64568, 70727

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn
• more

Comments

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

a(18) > 10^5. - Robert Price, Oct 19 2014

Links

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

Crossrefs

Cf. A002275, A056689, A093165.

Programs

Mathematica

Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}]

Formula

a(n) = A056689(n) + 1.

Extensions

a(13)-a(17) from Robert Price, Oct 19 2014