OFFSET
1,1
COMMENTS
Also, numbers k such that (691*10^k-7)/9 is prime.
Terms from Kamada.
a(7) > 30000.
LINKS
EXAMPLE
For k=3, 7*R_5 - 10^3 = 77777 - 1000 = 76777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(691*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((691*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved