OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 8 followed by the digits 67 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 68w67.
EXAMPLE
5 is in this sequence because (62*10^5 - 197)/9 = 6263 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 5, 688867;
a(3) = 7, 68888867;
a(4) = 14, 688888888888867;
a(5) = 17, 688888888888888867; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(62*10^# - 197)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
STATUS
approved