OFFSET
1,1
COMMENTS
For k > 2, numbers k such that k-6 occurrences of the digit 9 followed by the digits 199999 is prime (see Example section).
a(28) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 9w199999.
EXAMPLE
6 is in this sequence because 10^6 - 800001 = 199999 is prime.
Initial terms and associated primes:
a(1) = 6, 199999;
a(2) = 11, 99999199999;
a(3) = 38, 99999999999999999999999999999999199999;
a(4) = 43, 9999999999999999999999999999999999999199999; etc.
MATHEMATICA
Select[Range[6, 100000], PrimeQ[10^# - 800001] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 17 2017
STATUS
approved