OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 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 62w67.
EXAMPLE
1 is in this sequence because (56*10^1 + 403)/9 = 107 is prime.
Initial terms and associated primes:
a(1) = 1, 107;
a(2) = 7, 62222267;
a(3) = 8, 622222267;
a(4) = 11, 622222222267;
a(5) = 35, 622222222222222222222222222222222267; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(56 * 10^# + 403)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 31 2017
STATUS
approved