A056681 Numbers k such that 40*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 10, 27, 54, 93, 474, 2079, 4834, 5844, 12337, 20193, 49528, 56988, 71545, 77308

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (4*10^(k+1)-31)/9 is prime.

Links

• Makoto Kamada, Prime numbers of the form 44...441.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A099412.

Programs

Mathematica

Do[ If[ PrimeQ[40*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]

Formula

a(n) = A099412(n+1) - 1. - Robert Price, Jan 30 2015

Extensions

a(11)-a(16) derived from A099412 by Robert Price, Jan 30 2015