A056712 Numbers n such that 5*10^n-1 is prime.

2, 3, 4, 6, 14, 54, 210, 390, 594, 3460, 5028, 5219, 5332, 8072, 15796, 16131, 21456, 29282, 78790, 85142, 179972, 211029, 445773, 464843, 511056

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• hard
• nonn
• more

Comments

Also numbers n such that 4*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 499...99.
• Index entries for primes involving repunits

Programs

Magma

[n: n in [0..2000] |IsPrime(5*10^n-1)]; // Vincenzo Librandi, Oct 20 2014

Mathematica

Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]

PARI

is(n)=ispseudoprime(5*10^n-1) \\ Charles R Greathouse IV, Mar 01 2017

Extensions

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

a(21)-a(25) from Kamada data by Robert Price, Oct 19 2014