OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digits 22 followed by k-1 occurrences of the digit 3 followed by the digit 9 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 223w9.
EXAMPLE
4 is in this sequence because (67*10^4 + 17)/3 = 223339 is prime.
Initial terms and associated primes:
a(1) = 1, 229;
a(2) = 2, 2239;
a(3) = 4, 223339;
a(4) = 8, 2233333339;
a(5) = 11, 2233333333339; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(67*10^# + 17)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 19 2017
STATUS
approved