OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 9 followed by the digits 49 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 49w49.
EXAMPLE
2 is in this sequence because 5*10^2 - 51 = 449 is prime.
Initial terms and associated primes:
a(1) = 2, 449;
a(2) = 6, 4999949;
a(3) = 13, 49999999999949; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[5*10^# - 51] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 23 2017
STATUS
approved