A103032 Numbers n such that 6*10^n + 3*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 2, 4, 7, 18, 19, 23, 24, 34, 41, 56, 64, 84, 149, 272, 755, 1272, 2398, 2686, 4800, 5198, 6217, 8737, 12388, 12391, 19702, 42404

Offset: 1

Author

Robert G. Wilson v, Jan 18 2005

Keywords

• more
• nonn

Comments

Also numbers n such that (19*10^n-7)/3 is prime.

a(28) > 10^5. - Robert Price, Sep 10 2015

Links

• Makoto Kamada, Prime numbers of the form 633...331.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A101525.

Programs

Mathematica

Do[ If[ PrimeQ[(19*10^n - 7)/3], Print[n]], {n, 0, 10000}]

Formula

a(n) = A101525(n) + 1.

Extensions

Addition of a(24)-a(26) from Kamada data by Robert Price, Dec 09 2010

a(27) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013