OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 8 followed by the digits 01 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 68w01.
EXAMPLE
2 is in this sequence because (62*10^2 - 791)/9 = 601 is prime.
Initial terms and associated primes:
a(1) = 2, 601;
a(2) = 7, 68888801;
a(3) = 10, 68888888801;
a(4) = 34, 68888888888888888888888888888888801;
a(5) = 53, 688888888888888888888888888888888888888888888888888801; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(62*10^# - 791)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 27 2017
EXTENSIONS
a(21) from Robert Price, Jun 02 2019
STATUS
approved