OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 6 followed by the digits 81 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w81.
EXAMPLE
4 is in this sequence because (17*10^4 + 43)/3 = 56681 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 4, 56681;
a(3) = 5, 566681;
a(4) = 6, 5666681;
a(5) = 7, 56666681; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(17*10^# + 43)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 25 2017
STATUS
approved