A056716 Numbers n such that 6*10^n-1 is prime.

0, 1, 2, 4, 5, 7, 10, 13, 22, 23, 28, 34, 40, 61, 73, 361, 490, 613, 1624, 2000, 2994, 4301, 4332, 18668, 32544, 34936, 267598, 270658, 293134, 319889, 414508

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• hard
• nonn
• more

Comments

Also numbers n such that 5*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

Links

• Makoto Kamada, Prime numbers of the form 599...99.
• Index entries for primes involving repunits

Crossrefs

Cf. A056805 (6*10^n+1 is prime).

Programs

Mathematica

Do[ If[ PrimeQ[ 6*10^n - 1], Print[n]], {n, 0, 5000}]

PARI

is(n)=ispseudoprime(6*10^n-1) \\ Charles R Greathouse IV, Feb 20 2017

Extensions

More terms from Hugo Pfoertner, Feb 12 2004

3 more terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 30 2007

a(27)-a(31) from Kamada data by Robert Price, Oct 19 2014

Outdated comment deleted by Harvey P. Dale, Mar 01 2020