OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k - 367)/9 is prime.
Terms from Kamada.
a(9) > 30000.
a(9) > 2*10^5. - Tyler Busby, Feb 02 2023
LINKS
EXAMPLE
For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &]
PROG
(Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
STATUS
approved