OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 3 followed by the digits 51 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 73w51.
EXAMPLE
3 is in this sequence because (22*10^3 + 53)/3 = 7351 is prime.
Initial terms and associated primes:
a(1) = 2, 751;
a(2) = 3, 7351;
a(3) = 4, 73351;
a(4) = 5, 733351;
a(5) = 8, 733333351; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22*10^# + 53)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 09 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Jul 06 2019
STATUS
approved