A099417 Numbers k such that 5*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 3, 4, 6, 10, 15, 22, 88, 207, 528, 960, 2100, 18522, 36783, 38517

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn

Comments

Also numbers k such that (5*10^k + 13)/9 is prime.

a(17) > 10^5. - Robert Price, Oct 18 2014

Links

• Makoto Kamada, Prime numbers of the form 55...557.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056686.

Programs

Mathematica

Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 2], Print[n]], {n, 0, 5000}]

Formula

a(n) = A056686(n-1) + 1.

Extensions

a(14)-a(16) from Robert Price, Oct 18 2014