OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 4 followed by the digits 81 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 64w81.
EXAMPLE
3 is in this sequence because (58*10^3 + 329)/9 = 6481 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 101;
a(3) = 3, 6481;
a(4) = 6, 6444481;
a(5) = 19, 64444444444444444481; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(58*10^# + 329)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 02 2017
EXTENSIONS
a(24)-a(25) from Robert Price, May 01 2019
STATUS
approved