OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 6 followed by the digits 87 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 46w87.
EXAMPLE
4 is in this sequence because (14*10^4 + 61)/3 = 46687 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 487;
a(3) = 4, 46687;
a(4) = 12, 4666666666687;
a(5) = 15, 4666666666666687; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(14*10^# + 61)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 17 2017
EXTENSIONS
a(24)-a(27) from Robert Price, Dec 06 2018
STATUS
approved