OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digits 40 followed by k occurrences of the digit 9 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 409w.
EXAMPLE
2 is in this sequence because 41*10^2 - 1 = 409 is prime.
Initial terms and associated primes:
a(1) = 1, 409;
a(2) = 2, 4099;
a(3) = 4, 409999;
a(4) = 6, 40999999;
a(5) = 17, 4099999999999999999; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[41*10^# - 1] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
EXTENSIONS
a(21) from Robert Price, Jan 31 2020
STATUS
approved