OFFSET
1,2
COMMENTS
Also numbers k such that (25*10^k - 7)/9 is prime.
a(21) > 10^5. - Robert Price, Dec 17 2014
a(23) > 2*10^5. - Robert Price, May 19 2018
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 2*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(15) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(16)-a(20) from Robert Price, Dec 17 2014
a(21)-a(22) from Robert Price, May 19 2018
STATUS
approved