A056715 Numbers k such that 5*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 8, 14, 17, 18, 33, 35, 126, 183, 324, 344, 866, 992, 1226, 2355, 13344, 75027, 77322

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• hard
• nonn

Comments

Also numbers n such that (52*10^k - 7)/9 is prime.

a(20) > 10^5. - Robert Price, Dec 30 2014

Links

• Makoto Kamada, Prime numbers of the form 577...77.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093941.

Programs

Mathematica

Do[ If[ PrimeQ[ 5*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 5000}]

Extensions

a(17)-a(19) from Robert Price, Dec 30 2014