OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 79 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 16w79.
EXAMPLE
2 is in this sequence because (5*10^2 + 37) / 3 = 179 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 179;
a(3) = 5, 166679;
a(4) = 9, 1666666679;
a(5) = 13, 16666666666679; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^# + 37) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 16 2017
EXTENSIONS
a(26)-a(27) from Robert Price, Mar 02 2018
STATUS
approved