OFFSET
1,2
COMMENTS
Also numbers k such that 6*10^k + 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
a(35) > 3*10^5. - Robert Price, Jul 02 2023
LINKS
FORMULA
a(n) = A101541(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[7*10^n - 9], Print[n]], {n, 0, 10000}]
Select[Range[100000], PrimeQ[(7 10^# - 9)] &] (* Vincenzo Librandi, Oct 15 2015 *)
PROG
(Magma) [n: n in [1..500] | IsPrime(7*10^n-9)]; // Vincenzo Librandi, Oct 15 2015
(PARI) isok(n) = isprime(7*10^n-9 ); \\ Michel Marcus, Oct 15 2015
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Jan 18 2005
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
Addition of a(25)-a(32) from Kamada data by Robert Price, Dec 13 2010
a(33) from Robert Price, Oct 14 2015
a(34) from Robert Price, Jul 02 2023
STATUS
approved