
COMMENTS

Primes of the form (9*10^n  27)/9.  Vincenzo Librandi, Nov 16 2010
Also primes of the form 9*R_n  2, where R_n is the repunit (A002275) of length n.
The next term has 140 digits.
a(n) = 10^A089675(n)  3 = 10^(A056662(n) + 1)  3.  Farideh Firoozbakht, Nov 27 2013


MATHEMATICA

Do[If[PrimeQ[10^n  3], Print[10^n  3]], {n, 100}] (* Farideh Firoozbakht, Nov 27 2013 *)
Select[Table[FromDigits[PadLeft[{7}, n, 9]], {n, 25}], PrimeQ] (* Harvey P. Dale, Dec 12 2020 *)
