OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k - 187)/9 is prime.
Terms from Kamada.
a(17) > 30000.
a(20) > 2*10^5. - Tyler Busby, Feb 01 2023
LINKS
EXAMPLE
For k=4, 7*R_4 - 20 = 7777 - 20 = 7757 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(7*10^# - 187)/9] &]
PROG
(Magma) [n: n in [2..400] | IsPrime((7*10^n-187) div 9)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
EXTENSIONS
a(17)-a(19) from Tyler Busby, Feb 01 2023
STATUS
approved