OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 4 followed by the digits 73 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 24w73.
EXAMPLE
3 is in this sequence because (22*10^3 + 257)/9 = 2473 is prime.
Initial terms and associated primes:
a(1) = 1, 53;
a(2) = 3, 2473;
a(3) = 4, 24473;
a(4) = 6, 2444473;
a(5) = 22, 24444444444444444444473; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22*10^# + 257)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 12 2017
EXTENSIONS
a(21) from Robert Price, Jun 08 2018
STATUS
approved