OFFSET
1,1
COMMENTS
For k > 2, numbers k such that k-3 occurrences of the digit 9 followed by the digits 599 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 9w599.
EXAMPLE
6 is in this sequence because 10^6 - 401 = 999599 is prime.
Initial terms and associated primes:
a(1) = 3, 599;
a(2) = 6, 999599;
a(3) = 9, 999999599;
a(4) = 11, 99999999599;
a(5) = 12, 999999999599; etc.
MATHEMATICA
Select[Range[3, 100000], PrimeQ[10^# - 401] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 17 2017
EXTENSIONS
a(24) from Robert Price, Jan 19 2018
STATUS
approved