OFFSET
1,2
COMMENTS
Also, numbers k such that (91*10^k - 1)/9 is prime.
Terms from Kamada.
a(12) > 350000.
LINKS
EXAMPLE
For k=3, R_5 - 10^3 = 11111 - 1000 = 10111 which is prime.
MATHEMATICA
Select[Range[0, 350000], PrimeQ[(91*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((91*10^n - 1) div 9)]; // Vincenzo Librandi, Apr 13 2015
CROSSREFS
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 12 2015
STATUS
approved