OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 3 followed by the digits 09 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 73w09.
EXAMPLE
3 is in this sequence because (22*10^3 - 73)/3 = 7309 is prime.
Initial terms and associated primes:
a(1) = 2, 709;
a(2) = 3, 7309;
a(3) = 4, 73309;
a(4) = 17, 733333333333333309;
a(5) = 34, 73333333333333333333333333333333309; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(22*10^# - 73)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
EXTENSIONS
a(24) from Robert Price, Apr 20 2019
STATUS
approved