OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 6 followed by the digits 89 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w89.
EXAMPLE
3 is in this sequence because (17*10^3 + 67)/3 = 5689 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 3, 5689;
a(3) = 7, 56666689;
a(4) = 8, 566666689;
a(5) = 9, 5666666689; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(17*10^# + 67)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 09 2017
EXTENSIONS
a(24)-a(26) from Robert Price, Jan 24 2019
STATUS
approved