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 57 is prime (see Example section).
a(25) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w57.
EXAMPLE
3 is in this sequence because (17*10^3 - 29)/3 = 5657 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 557;
a(3) = 3, 5657;
a(4) = 9, 5666666657;
a(5) = 10, 56666666657; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(17*10^# - 29)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 05 2017
STATUS
approved