OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 91 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 13w91.
EXAMPLE
2 is in this sequence because (4*10^2 + 173) / 3 = 191 is prime.
Initial terms and associated primes:
a(1) = 0, 59;
a(2) = 1, 71;
a(3) = 2, 191;
a(4) = 5, 133391;
a(5) = 7, 13333391; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(4*10^# + 173) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 09 2017
EXTENSIONS
a(29)-a(30) from Robert Price, Feb 16 2018
STATUS
approved