OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 69 followed by k-1 occurrences of the digit 6 followed by the digit 1 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 696w1.
EXAMPLE
3 is in this sequence because (209*10^3 - 17)/3 = 69661 is prime.
Initial terms and associated primes:
a(1) = 1, 691;
a(2) = 2, 6961;
a(3) = 3, 69661;
a(4) = 7, 696666661;
a(5) = 13, 696666666666661; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(209*10^# - 17)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 03 2017
STATUS
approved