OFFSET
1,2
COMMENTS
Also numbers k such that (88*10^k - 7)/9 is prime.
a(21) > 10^5. - Robert Price, Nov 30 2014
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 9*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(18)-a(20) from Robert Price, Nov 30 2014
STATUS
approved