A099421 0 together with numbers k such that 8*R_k - 7 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 3, 19, 79, 139, 223, 463, 544, 1096, 1419, 3247, 3877, 4417, 9507, 11091, 14602, 27811, 29188, 106729, 188308

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn

Comments

Also numbers k such that abs(8*10^k - 71)/9 is a prime.

a(19) > 10^5. - Robert Price, Sep 06 2014

Links

• Makoto Kamada, Prime numbers of the form 88...881.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056664.

Programs

Mathematica

Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 7], Print[n]], {n, 0, 15000}]

PARI

for(n=0,10^4,if(ispseudoprime(abs(8*(10^n-1)/9-7)),print1(n,", "))) \\ Derek Orr, Sep 06 2014

Formula

a(n) = A056664(n-1) + 1.

Extensions

a(17)-a(18) from Kamada data by Robert Price, Sep 06 2014

a(19)-a(20) from Kamada data by Tyler Busby, Apr 30 2024