OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 61 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 42w61.
EXAMPLE
2 is in this sequence because (38*10^2 + 349)/9 = 461 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 2, 461;
a(3) = 3, 4261;
a(4) = 8, 422222261;
a(5) = 27, 4222222222222222222222222261; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(38*10^# + 349)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 04 2017
EXTENSIONS
a(23)-a(24) from Robert Price, Oct 01 2018
STATUS
approved