OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 65 followed by k-1 occurrences of the digit 6 followed by the digit 3 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 656w3.
EXAMPLE
2 is in this sequence because (197*10^2 - 11)/3 = 6563 is prime.
Initial terms and associated primes:
a(1) = 1, 653;
a(2) = 2, 6563;
a(3) = 5, 6566663;
a(4) = 11, 6566666666663;
a(5) = 24, 65666666666666666666666663; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(197*10^# - 11)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
EXTENSIONS
a(23) from Robert Price, Mar 22 2020
STATUS
approved