OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 2 followed by the digits 67 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 52w67.
EXAMPLE
4 is in this sequence because (47*10^4 + 403)/9 = 52267 is prime.
Initial terms and associated primes:
a(1) = 1, 97;
a(2) = 4, 52267;
a(3) = 10, 52222222267;
a(4) = 12, 5222222222267;
a(5) = 16, 52222222222222267; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(47*10^# + 403)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 21 2017
EXTENSIONS
a(24) from Robert Price, Mar 07 2019
STATUS
approved