OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 81 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 89w81.
EXAMPLE
5 is in this sequence because 9*10^5 - 19 = 899981 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 2, 881;
a(3) = 5, 899981;
a(4) = 6, 8999981;
a(5) = 7, 89999981; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[9*10^# - 19] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 04 2017
EXTENSIONS
a(25) from Robert Price, Jul 27 2019
STATUS
approved