OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 69 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
a(21) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 690w1.
EXAMPLE
3 is in this sequence because 69*10^3 + 1 = 69001 is prime.
Initial terms and associated primes:
a(1) = 1, 691;
a(2) = 3, 69001;
a(3) = 5, 6900001;
a(4) = 15, 69000000000000001;
a(5) = 17, 6900000000000000001; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[69*10^# + 1] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 27 2017
STATUS
approved