A102930 Numbers k such that 10^k + 2*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 4, 14, 25, 32, 56, 62, 811, 812, 1084, 2533, 11924, 49552

Offset: 1

Author

Robert G. Wilson v, Dec 16 2004

Keywords

• more
• nonn

Comments

Also numbers k such that (11*10^k + 43)/9 is prime.

a(15) > 10^5. - Robert Price, Jan 16 2015

a(15) > 2*10^5. - Tyler Busby, Feb 01 2023

Links

• Makoto Kamada, Prime numbers of the form 122...227.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A102010.

Programs

Mathematica

Do[ If[ PrimeQ[(11*10^n + 43)/9], Print[n]], {n, 0, 10000}]

Formula

a(n) = A102010(n) + 1.

Extensions

Addition of a(13) from Kamada data by Robert Price, Dec 12 2010

a(14) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013