A056720 Numbers k such that 7*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 3, 5, 53, 56, 343, 908, 1079, 2204, 2379, 9134, 9371, 9728, 11039, 11247, 13275, 16781, 20138, 75368, 87863

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (22*10^k - 1)/3 is prime.

a(23) > 10^5. - Robert Price, Dec 26 2014

Links

• Makoto Kamada, Prime numbers of the form 733...33.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093675.

Programs

Mathematica

Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(16)-a(22) from Robert Price, Dec 26 2014