OFFSET
1,2
COMMENTS
Also, numbers k such that 2*10^k - 9 is prime.
Terms from Kamada data.
a(12) > 2*10^5.
LINKS
EXAMPLE
For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.
MATHEMATICA
Select[Range[200000], PrimeQ[2*10^n-9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved