OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 6 followed by the digits 99 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 76w99.
EXAMPLE
6 is in this sequence because (23*10^6 + 97)/3 = 7666699 is prime.
Initial terms and associated primes:
a(1) = 3, 7699;
a(2) = 6, 7666699;
a(3) = 11, 766666666699;
a(4) = 14, 766666666666699;
a(5) = 20, 766666666666666666699; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(23*10^# + 97)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 10 2017
STATUS
approved