OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 55 followed by k-2 occurrences of the digit 6 followed by the digit 1 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 556w1.
EXAMPLE
3 is in this sequence because (167*10^3 - 17)/3 = 55661 is prime.
Initial terms and associated primes:
a(1) = 3, 55661;
a(2) = 5, 5566661;
a(3) = 13, 556666666666661;
a(4) = 15, 55666666666666661;
a(5) = 21, 55666666666666666666661; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(167*10^# - 17)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 18 2017
EXTENSIONS
a(21)-a(23) from Robert Price, Mar 27 2020
STATUS
approved