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 49 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 45w49.
EXAMPLE
3 is in this sequence because (41*10^3 - 59)/9 = 4549 is prime.
Initial terms and associated primes:
a(1) = 2, 449;
a(2) = 3, 4549;
a(3) = 11, 455555555549;
a(4) = 14, 455555555555549;
a(5) = 15, 4555555555555549; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(41*10^# - 59)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 16 2017
EXTENSIONS
a(24) from Robert Price, Oct 27 2018
STATUS
approved