OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 22 followed by k-2 occurrences of the digit 3 followed by the digit 1 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 223w1.
EXAMPLE
5 is in this sequence because (67*10^5 - 7)/3 = 2233331 is prime.
Initial terms and associated primes:
a(1) = 4, 223331;
a(2) = 5, 2233331;
a(3) = 9, 22333333331;
a(4) = 20, 2233333333333333333331;
a(5) = 21, 22333333333333333333331; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(67*10^# - 7)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 06 2017
EXTENSIONS
a(22) from Robert Price, Jan 01 2020
STATUS
approved