OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 6 followed by the digits 09 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 86w09.
EXAMPLE
2 is in this sequence because (26*10^2 - 173)/3 = 809 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 809;
a(3) = 3, 8609;
a(4) = 8, 866666609;
a(5) = 11, 866666666609; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(26*10^# - 173)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 19 2017
STATUS
approved