OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 69 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 95w69.
EXAMPLE
4 is in this sequence because (86*10^4 + 121)/9 = 95569 is prime.
Initial terms and associated primes:
a(1) = 0, 23;
a(2) = 1, 109;
a(3) = 4, 95569;
a(4) = 6, 9555569;
a(5) = 24, 9555555555555555555555569; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(86*10^# + 121)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 02 2017
EXTENSIONS
a(28) from Robert Price, Aug 30 2019
STATUS
approved