A103001 Numbers k such that 5*10^k - 9 is prime.

1, 2, 4, 7, 24, 172, 173, 218, 439, 2563, 2782, 2880, 3715, 4392, 10820, 14592, 25054, 27769, 40168, 180992, 193907

Offset: 1

Author

Robert G. Wilson v, Jan 17 2005

Keywords

• more
• nonn

Comments

Also numbers k such that 4*10^k + 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

a(22) > 2*10^5. - Robert Price, May 30 2015

Links

• Makoto Kamada, Prime numbers of the form 499...991.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A101738.

Programs

Mathematica

Do[ If[ PrimeQ[5*10^n - 9], Print[n]], {n, 0, 10000}]

PARI

is(n)=ispseudoprime(5*10^n-9) \\ Charles R Greathouse IV, Jun 06 2017

Formula

a(n) = A101738(n) + 1.

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(17)-a(19) from Kamada data added by Robert Price, Dec 10 2010

a(20)-a(21) from Robert Price, May 30 2015