OFFSET
1,1
COMMENTS
Also numbers k such that (79*10^k - 7)/9 is prime.
a(18) > 10^5. - Robert Price, Jan 11 2015
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 8*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
a(16)-a(17) from Robert Price, Jan 11 2015
STATUS
approved