OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k - 2 occurrences of the digit 3 followed by the digits 27 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 73w27.
EXAMPLE
4 is in this sequence because (22*10^4 - 19)/3 = 73327 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 727;
a(3) = 4, 73327;
a(4) = 6, 7333327;
a(5) = 14, 733333333333327; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(22 * 10^# - 19)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 06 2017
EXTENSIONS
a(28) from Robert Price, Apr 27 2019
a(29)-a(30) from Robert Price, Oct 26 2023
STATUS
approved