OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digits 17 followed by k occurrences of the digit 3 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 173w.
EXAMPLE
3 is in this sequence because (52*10^3 - 1)/3 = 17333 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 1, 173;
a(3) = 2, 1733;
a(4) = 3, 17333;
a(5) = 8, 1733333333; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(52*10^# - 1)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 16 2017
STATUS
approved