OFFSET
1,3
COMMENTS
Also numbers k such that (22*10^k - 1)/3 is prime.
a(23) > 10^5. - Robert Price, Dec 26 2014
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(16)-a(22) from Robert Price, Dec 26 2014
STATUS
approved