OFFSET
1,1
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
MATHEMATICA
Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]
PROG
(Magma) [n: n in [0..2000] |IsPrime(5*10^n-1)]; // Vincenzo Librandi, Oct 20 2014
(PARI) is(n)=ispseudoprime(5*10^n-1) \\ Charles R Greathouse IV, Mar 01 2017
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Robert G. Wilson v, Aug 11 2000
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
STATUS
approved