OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 5 followed by the digits 19 is prime (see Example section).
a(19) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 45w19.
EXAMPLE
2 is in this sequence because (41*10^2 - 329)/9 = 419 is prime.
Initial terms and associated primes:
a(1) = 2, 419;
a(2) = 3, 4519;
a(3) = 9, 4555555519;
a(4) = 18, 4555555555555555519;
a(5) = 21, 4555555555555555555519; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(41*10^# - 329)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 03 2017
STATUS
approved