OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 4 followed by the digits 53 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 84w53.
EXAMPLE
2 is in this sequence because (76*10^2 + 77)/9 = 853 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 2, 853;
a(3) = 5, 844453;
a(4) = 9, 8444444453;
a(5) = 24, 8444444444444444444444453; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(76*10^# + 77)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 05 2017
EXTENSIONS
a(21)-a(23) from Robert Price, Sep 30 2019
STATUS
approved