A099412 0 together with numbers k such that 4*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 4, 11, 28, 55, 94, 475, 2080, 4835, 5845, 12338, 20194, 49529, 56989, 71546, 77309, 284330

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn

Comments

Also numbers k such that abs((4*10^k - 31))/9 is prime.

a(18) > 10^5. - Robert Price Sep 06 2014

Links

• Makoto Kamada, Prime numbers of the form 44...441.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056681, A104659.

Programs

Mathematica

Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 3], Print[n]], {n, 0, 7000}]

PARI

for(n=0,10^4,if(ispseudoprime(abs(4*(10^n-1)/9-3)),print1(n,", "))) \\ Derek Orr, Sep 06 2014

Formula

a(n) = A056681(n-1) + 1.

Extensions

a(12)-a(17) from Kamada data by Robert Price, Sep 06 2014

a(18) from Kamada data by Tyler Busby, Apr 30 2024