OFFSET
1,1
COMMENTS
Also numbers k such that the concatenation (k 9's)1(k-1 9's) is prime.
EXAMPLE
3 is a term because 999199 is prime.
4 is a term because 99991999 is prime.
MATHEMATICA
Select[Range[500], PrimeQ[10^(2*#) - 8*10^(#-1) - 1] &] (* Amiram Eldar, Jan 28 2020 *)
PROG
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Eder Vanzei, Jan 27 2020
EXTENSIONS
a(7) from Giovanni Resta, Jan 28 2020
STATUS
approved