A056703 Numbers n such that 3*10^n - 1 is prime.

0, 1, 3, 6, 7, 19, 27, 43, 55, 207, 1311, 3204, 7050, 9439, 26044, 33058, 34507, 49314, 119292

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn
• more

Comments

Also numbers n such that 2*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 299...99.
• Index entries for primes involving repunits

Programs

Mathematica

Do[ If[ PrimeQ[ 2*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]
Select[Range[200000],PrimeQ[3*10^#-1]&] (* Harvey P. Dale, Sep 29 2012 *)

PARI

is(n)=ispseudoprime(3*10^n-1) \\ Charles R Greathouse IV, Jun 12 2017

Extensions

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