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 51 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 56w51.
EXAMPLE
3 is in this sequence because (17*10^3 - 47)/3 = 5651 is prime.
Initial terms and associated primes:
a(1) = 1, 41;
a(2) = 3, 5651;
a(3) = 8, 566666651;
a(4) = 9, 5666666651;
a(5) = 14, 566666666666651; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(17*10^# - 47)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 03 2017
STATUS
approved