A056682 Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 3, 9, 19, 25, 721, 1309, 3169, 28933, 66283, 67795

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (4*10^(k+1)+23)/9 is prime.

a(13) > 10^5. - Robert Price, Nov 30 2014

Links

• Makoto Kamada, Prime numbers of the form 44...447.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A099413.

Programs

Mathematica

Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

Formula

a(n) = A099413(n+1)-1. - Robert Price, Nov 30 2014

Extensions

a(10)-a(12) derived from A099413 by Robert Price, Nov 30 2014