A056686 Numbers k such that 50*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 5, 9, 14, 21, 87, 206, 527, 959, 2099, 18521, 36782, 38516

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (5*10^(k+1)+13)/9 is prime.

a(16) > 10^5. - Robert Price, Nov 01 2014

Links

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

Crossrefs

Cf. A002275, A099417.

Programs

Mathematica

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

Formula

a(n) = A099417(n+1) - 1. - Robert Price, Nov 01 2014

Extensions

a(13)-a(15) derived from A099417 by Robert Price, Nov 01 2014