OFFSET
1,1
COMMENTS
Equivalently, primes of the form 7*10^n + 9*R_n, where R_n is the repunit (A002275) of length n.
LINKS
MATHEMATICA
Select[Table[FromDigits[PadRight[{7}, n, 9]], {n, 50}], PrimeQ] (* Vincenzo Librandi, May 08 2019 *)
PROG
(Magma) [a: n in [0..200] | IsPrime(a) where a is (8*10^n-1)]; // Vincenzo Librandi, May 08 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Rick L. Shepherd, Apr 17 2004
STATUS
approved