A056723 Numbers k such that 8*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 7, 23, 29, 133, 173, 367, 1925, 3707, 5765, 9709, 19573, 43753

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• nonn

Comments

Also numbers k such that (25*10^k - 1)/3 is prime.

Links

• Makoto Kamada, Prime numbers of the form 833...33.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093676.

Programs

Mathematica

Do[ If[ PrimeQ[ 8*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 10000}]

Extensions

Corrected by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

19573 and 43753 from Serge Batalov, Jan 2009 confirmed as next terms by Ray Chandler, Feb 13 2012