OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 43w69.
EXAMPLE
5 is in this sequence because (13*10^5 + 107)/3 = 433369 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 5, 433369;
a(3) = 7, 43333369;
a(4) = 9, 4333333369;
a(5) = 16, 43333333333333369; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(13*10^# + 107)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
STATUS
approved