OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 8 followed by the digits 69 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 98w69.
EXAMPLE
2 is in this sequence because (89*10^4 - 179)/9 = 98869 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 4, 98869;
a(3) = 10, 98888888869;
a(4) = 12, 9888888888869;
a(5) = 15, 9888888888888869; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(89*10^# - 179)/9] &]
PROG
(PARI) isok(k) = isprime((89*10^k - 179)/9); \\ Michel Marcus, Nov 15 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 15 2017
EXTENSIONS
a(23)-a(24) from Robert Price, Dec 16 2019
STATUS
approved