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 39 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 76w39.
EXAMPLE
3 is in this sequence because (23*10^3 - 83)/3 = 7639 is prime.
Initial terms and associated primes:
a(1) = 2, 739;
a(2) = 3, 7639;
a(3) = 5, 766639;
a(4) = 6, 7666639;
a(5) = 16, 76666666666666639; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(23*10^# - 83)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Aug 12 2019
STATUS
approved