OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 4 followed by the digits 89 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 14w89.
EXAMPLE
3 is in this sequence because (13*10^3 + 401)/9 = 1489 is prime.
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 3, 1489;
a(3) = 4, 14489;
a(4) = 6, 1444489;
a(5) = 7, 14444489; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(13*10^# + 401)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 09 2017
EXTENSIONS
a(23) from Robert Price, Jan 08 2018
STATUS
approved