OFFSET
1,3
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
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 *)
PROG
(PARI) is(n)=ispseudoprime(3*10^n-1) \\ Charles R Greathouse IV, Jun 12 2017
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
STATUS
approved