OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 69 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 18w69.
EXAMPLE
5 is in this sequence because (17*10^5 - 179)/9 = 188869 is prime.
Initial terms and associated primes:
a(1) = 4, 18869;
a(2) = 5, 188869;
a(3) = 10, 18888888869;
a(4) = 13, 18888888888869;
a(5) = 16, 18888888888888869; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(17*10^# - 179)/9] &]
PROG
(PARI) is(n)=isprime((17*10^k - 179)/9) \\ Charles R Greathouse IV, Mar 05 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 05 2017
STATUS
approved