A056661 Numbers k such that 40*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 5, 8, 11, 29, 31, 182, 296, 491, 41315

Offset: 1

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• hard
• more
• nonn

Comments

Also numbers k such that (4*10^(k+1)-13)/9 is prime.

a(13) > 10^5. - Robert Price, Nov 01 2014

Links

• Makoto Kamada, Prime numbers of the form 44...443.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A096845.

Programs

Mathematica

Do[ If[ PrimeQ[ 40*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

Formula

a(n) = A096845(n) - 1. - Robert Price, Nov 01 2014

Extensions

a(12) derived from A096845 by Robert Price, Nov 01 2014