OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 6 followed by the digits 89 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 96w89.
EXAMPLE
3 is in this sequence because (29*10^3 + 67)/3 = 9689 is prime.
Initial terms and associated primes:
a(1) = 3, 9689;
a(2) = 6, 9666689;
a(3) = 8, 966666689;
a(4) = 10, 96666666689;
a(5) = 16, 96666666666666689; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(29*10^# + 67)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 09 2017
EXTENSIONS
a(21)-a(22) from Robert Price, Dec 18 2019
STATUS
approved