OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 67 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 42w67.
EXAMPLE
2 is in this sequence because (38*10^82 2 161+403)/9 = 467 is prime.
Initial terms and associated primes:
a(1) = 2, 467;
a(2) = 5, 422267;
a(3) = 8, 422222267;
a(4) = 11, 422222222267;
a(5) = 15, 4222222222222267; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(38*10^# + 403)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 24 2017
EXTENSIONS
a(21) from Robert Price, Oct 03 2018
STATUS
approved