A099411 Numbers k such that 3*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 6, 46, 394, 978, 2586, 2811, 2968, 3642, 4827, 4918, 5592, 5706, 10683, 12891, 14118, 74350, 88680, 162138, 279978

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• hard
• nonn

Comments

Also numbers k such that (10^k + 11)/3 is prime.

a(21) > 10^5. - Robert Price, Nov 02 2014

Links

• Makoto Kamada, Prime numbers of the form 33...337.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056680, A093168.

Programs

Mathematica

Do[ If[ PrimeQ[ 3(10^n - 1)/9 + 4], Print[n]], {n, 10000}]

Formula

a(n) = A056680(n) + 1.

Extensions

a(16)-a(20) from Robert Price, Nov 02 2014

a(21)-a(22) from Kamada data by Tyler Busby, May 03 2024