OFFSET
1,2
COMMENTS
Also, numbers k such that (709*10^k - 7)/9 is prime.
Terms from Kamada.
a(22) > 2*10^5.
LINKS
EXAMPLE
For k=4, 7*R_6 + 10^4 = 777777 + 10000 = 787777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(709*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((709*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
EXTENSIONS
a(21) from Robert Price, Sep 28 2017
STATUS
approved