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 53 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w53.
EXAMPLE
3 is in this sequence because (17*10^3 - 41)/3 = 5653 is prime.
Initial terms and associated primes:
a(1) = 1, 43;
a(2) = 3, 5653;
a(3) = 5, 566653;
a(4) = 7, 56666653;
a(5) = 13, 56666666666653; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(17*10^# - 41)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 04 2017
STATUS
approved